Every once in a while, I download several game demos or take advantage of a Steam weekend sale, and try a bunch of games. This gives me a chance to keep up on what others are doing and learn from their example, both things I should and shouldn't do. There probably isn't any game developer that would say that good game design is not important, but actions speak louder than words, and many of the games I play have some glaring design problems.
Of course, much of the field of game design is simply opinion, so just because I think a game has a "glaring design problem" doesn't mean it really has a problem or that it won't sell. Even if I'm pickier than the average potential customer, there are players out there like me, and incorporating the following guidelines would probably help.
1. Only make interactive those parts of the story that should be interactive.
Use short cut scenes for the rest. Cut scenes do serve an important purpose. They define the setting, flesh out characters, explain story elements that would be difficult to convey with gameplay, they provide rest after intense play sequences, etc. Those are good things, as long as they're kept short, because the player isn't playing a game to watch cut scenes.
That said, it is possible to go too far the other direction: making some parts of the game interactive when they shouldn't be. The original Assassin's Creed is the best example of this flaw I can think of. In between sessions on the table, you actually have to control your character to get off the table, walk into his bedroom, and press the "use" key on his bed. The game seriously won't continue until you do that. If you take too long, say, because you think there's a reason you're actually controlling the character, so you start to explore the lab, the man just keeps saying stuff like, "Go to your room to rest."
After going to bed, the screen turns black, and then you're waking up. Your guy stands up by the bed and then waits for you to take over. The guy in the lab starts telling you to get on the table. Yep, that's right. You have to walk all the way in there and press the "use" key to get on the table.
There is one part I remember where you can talk to the guy's female assistant and get some info, but all that boring walking for that little story element? Not worth it. The player should only be doing interesting things. Less interesting things should not even be in the game, but if they really are necessary for some reason, they should only be briefly covered in a cut scene.
This is the reason I never even tried WoW. Whenever I saw my brothers playing, they were doing one of two things: running or waiting. They were running from one part of the world to the next, just running, running, running, and more running. I was playing a different game on my computer, and whenever I looked at my brother's screen, he was still running. If they weren't running, then they were waiting for people to join their group for a raid. That was years ago, and I know the game has changed a lot, but for me, grinding in general is a problem. More on this below.
2. The coolest and/or most critical parts of the game should not be in the cut scenes.
I first learned this design principle from Warcraft III. There's a character who turns evil and becomes a Death Knight or something. At one point in the cut scene, he charges out through a crowd of soldiers, and he 1-hit kills a bunch of guys in his way—BAM, BAM, BAM, they fall with every stroke of his arm, and he road away untouched. It was awesome. I haven't touched the game in several years, and that's all I remember about the cut scenes. Then in the game, you have him as your Death Knight hero. Can you do that with him? Nope. He stands there and hacks at the same enemy soldier over and over again like an impotent 2 year-old. Disappointing. But it sure was an awesome cut scene!
Assassin's Creed made this error too. I played about three missions into the game, and each one went as follows. You work your way through town, and you finally get to the target's lair. The only way in is through the front door (when do assassins use the front door?). As soon as you enter the doorway, you're ripped out the game experience and thrown into a cut scene. You proceed to watch your guy call out his target (when do assassins do that?), talk smack, the target tries to recruit you to his team, and then talks smack back when you refuse. And then after stirring up a beehive of angry bees and plopping you in the middle of it, the game gives you control of your guy again. Every time that happened, I could hear the designers laughing at the ridiculous situation they got me into during that cut scene. If I could have controlled my guy, I certainly would have done things differently.
Let the player do the cool stuff and make the critical decisions.
3. Read Raph Koster's Theory of Fun, and apply it.
In that book, I think Raph hits the nail on the head. Games are fun because they tap into our natural desire to learn and master new skills. Games can tap into other natural desires, like curiosity, when we play to find out what happens in the story, find out what gadget X does, or to find out what's on the other side of that mountain. Movies and books also tap into that curiosity, but only games can do the former.
My #1 reason for uninstalling a game I didn't finish (which is most of them) is that I just get bored. I've never been hooked by a game's story, and I'm rarely intrigued enough by the gadgets and abilities to play just to find out what they do (not because I'm smarter or more refined and crap—it's just me, my preference). The main reason I play is to do something new. Games which aren't continually requiring the player to improve their skills, learn new skills, and apply their skills in new ways miss out on this most compelling reason to play a game at all. Again, there are other reasons players might play a game, but the skill angle is unique to games.
There are many popular games that have this problem. Those other reasons players play games might still be sufficient for them to have fun, finish the game, and recommend it to their friends. But I submit that those players would have loved the game even more if it kept up the skill learning.
Requiring the player to continually learn and improve his skills doesn't mean the game needs to be really hard. It just means that the gameplay needs to keep evolving. Games are hard when the incremental steps that are required of the player are far apart. Games are boring when those steps disappear or even almost disappear.
FPS games frequently have this problem. You encounter harder, stronger enemies, and you get bigger, stronger guns to kill them with, but the core gameplay of "run, aim, fire, and keep firing until everyone's dead" stays the same. Usually a shooter will incorporate some new mechanics which set it apart from other games in the genre, but then the game still needs to evolve and stay fresh throughout. Usually, if the new features of the game are compelling enough, I'll play for an hour or so, but by then I've gotten a handle on them, and nothing new happens, so I'm done. Or I'll play for an hour and never get past the same old stuff and conclude the new stuff isn't worth my time to even try.
The FPS genre is one of my favorites to play, but I have only finished 2 FPS single player games: Far Cry (the original) and The Chronicles of Riddick: Assault on Dark Athena. I finished the former because the game kept giving me new situations to solve and requiring me to push my skills to new levels. I played Dark Athena a few weeks ago, and while I had other issues with it (the melee combat and ridiculously gross dialogue by Jaylor, to name two), it let me do some awesome stuff, like drive a mech and control the drones (the Alpha Drone was particularly cool). Even while not controlling those things, the weapons had different uses and required different play-styles, and that kept the game fresh (I'm a sci-fi fan too, and that never hurts).
The defining core mechanics of other mainstream genres, e.g. RTS and RPG, are also done to death, and I don't play through those single-player games either. In the case of the latter, grinding isn't fun because it's the same thing over and over again. I've never played an RPG that didn't include grinding as part of the game. It might be part of the genre's definition though—the slot machine effect: you're dumping minute after minute of your life into the game, hoping the next monster will drop something good. I have no use for that as a player, and I wouldn't enjoy making a game like that either.
It's hard to make a game that continually stays fresh. I don't know if we've succeeded at it, because by the time I was done with Now Boarding, I couldn't bear loading another airplane. I can't objectively judge the final product. As employees take over the mundane tasks of grouping passengers and loading, docking, and undocking airplanes, the gameplay does evolve. The different maps offer different routing problems to solve. But it's only $15, so it doesn't need to last for weeks before players grok it.
An RPG that is a long quest would be a ton of work to make it feel epic and stay interesting from a skill perspective. Maybe development costs would be prohibitive, but anything developers can do to require new and different skills whenever they can would improve the game.
4. Give the player clear short-term goals.
Clear means that the player understands what he is supposed to do and can reasonably figure out how to do it from what he's learned about the game world to that point.
Short-term means that the goal won't take too many steps to accomplish. Finishing missions is a natural hook for players, so let them do it—and keep doing it. Don't go too far and make the goals too short and small (unless you're developing a game like Achievement Unlocked). I like to finish what I'm doing before I quit playing, and I don't want to be locked in for hours. It's nice when the game can be enjoyed in small bites. I can always eat more bites in one sitting if I want to.
Not knowing what to do or how to do it is probably my #2 reason for uninstalling games. Most games are pretty good about this, but occasionally I run into a situation where I'm stuck. I go over past dialogue (if it's available), I retrace my steps and look for things I missed, but once I exhaust my resources in the game and/or my patience, I'm done. Alt-F4. And if Alt-F4 doesn't instantly close the game, then I instantly uninstall it when I finally get out. If Alt-F4 works and the game was otherwise compelling, I might google for a walk-through or something.
The most extreme example of not giving the player enough information is Silent Hunter 3. It's so bad I had to find walkthroughs just to explain the main menu. The reason is it's hardly even a game, but rather just a simulation tool. Usually a single player game has some kind of guided experience, i.e. you click on "Play" or "Career" or "Campaign" or something like that, and then you play through the game, and it gets increasingly difficult. In SH3, the "Career" option doesn't let you play anything (I never learned what it was for. I created a new profile with my name, but I couldn't select it or anything). The single player game is actually just a list of scenarios, each with several load-out options. All are available from the start, so there's no way to be sure you're playing the "right one" when you first play. After reading through a walkthrough to find out how the missions work, I played through the missions, which were quite fun (well, excpept the Gibraltar mission which was ridiculously long and tedious...I don't think I have the patience to be a real U-boat captain).
Aquaria also lost me for this reason. Where do I go? What do I do? How do I unlock that door? I have no idea. It's a beautiful game visually, but I hardly got to see any of it. Other artsy games like The Path and Blueberry Garden don't give the player clear goals either (well, The Path seemed to: "Stay on the path." I did, and it was short and boring, and I got 0's for all my scores. It lied to me. :) ). It's great that other people like those kinds of games. I just can't get into them. Exploration and experimentation are not reasons I play video games. I do a lot of that in the real world. But again, that's just me. If you make a game that encourages and rewards exploration and experimentation, why not add some goals in there too? You don't need to give the goals time pressure or anything that would hamper the explorer's experience, but then people like me would like your game too.
These are just four major things I've picked up over the years. All games have more specific issues that could have been done better and which are useful to talk about, but I wanted this post to be generally useful. Most games could be improved in these areas, and if thought is given to these from the beginning of a project, they probably wouldn't require any more work.
I think it's sad when a game really stinks. A game represents a lot of time and creative effort, the expenditure of real human life value. Usually, the difference in work between a crappy game and a good game is a relatively few small changes. But when those changes are left undone, all that effort is wasted.
 About the same time I played Assassin's Creed, I also played Hitman: Blood Money. That is an assassin game done right, and I played every level until I got the "Silent Assassin" rating. Assassin's Creed should have been called Pompous Brawler's Creed.
 There are other reasons people play games too. MMORPG's tap into our social natures, and games can also stimulate our OCD tendencies, e.g. when I have to finish a collection or find all the hidden packages or whatever. Some players also play games to escape reality, and that's sad.
 Multiplayer games have the inherent ability to push players and keep their skills evolving, but they are difficult and expensive to make. Multiplayer is the reason I like FPS games so much. I've played a lot of Counter-Strike, Team Fortress Classic, Team Fortress 2, Combat Arms, and Left 4 Dead. The most successful games have been multiplayer ones, but they're more of a "cash dragon" than a "cash cow"—if you can manage to slay the dragon, you'll make a lot of money.
 Grok is the term Raph Koster uses to describe when a player has mastered the skills of a game. This is the point at which I usually quit playing.
 The key term is "relatively". If you think about a game project, there's a lot of art and a lot of code that went into it. As an example, it wouldn't have been much more work to make the "Career" mode in SH3 take the player through each scenario with the recommended load-out. In the final stages of Now Boarding development and now Clockwords development, little changes that require very little work end up making a big difference in the overall player experience.