Multiplayer Doesn't Count
First off, this article is only concerned with the solo experiences offered in games. Multiplayer modes obviously add value and gameplaly hours to a game. We're only worried about single-player value here.
Replay Value Is King
Some games are actually intended to be played through more than once because on your first playthough you probably aren't really seeing everything the game has to offer. Or you only have a fraction of the combos. Or, frankly, you don't really know what you're doing your first time through. These types of games usually have a New Game+ option or at the very least your moves and abilities carry over so you can re-play the levels. When you re-play a level with enhanced abilities and combos, the game suddenly becomes much deeper and more interesting and more fun.
Two great examples of this from the last year are Lollipop Chainsaw and Metal Gear Rising: Revengeance. The main complaint people had about these games was that they are only 4-5 hours long. If you are only playing them once, however, you are absolutely doing it wrong. You don't even unlock the good stuff in Lollipop Chainsaw until your second (or third) playthrough, and there are enough hidden weapons and costumes and neat secret stuff (along with a ton of moves and abilities to upgrade) in Revengeance that you're only experiencing a fraction of what the game has to offer by only playing it once. You really can't complain about game length if you're the one doing it wrong to begin with.
This actually applies to longer games as well that people insist on trying to play through once before shelving it. The first Dead Rising, for example, takes exactly 6 hours to beat (because that is how long the 72-hour in-game time limit lasts in real time), so it isn't a short game. You are not, however, expected to run through it and beat the story on your first time through the game. You are actually supposed to level up, and die, and restart (while carrying over your skills from before you died) a few times to build your character up a bit before you try to beat the story. People complained that the game was too difficult, but that is only because they weren't playing it right. You can level up to level 50, so why do you expect to beat the game at only level 10 or less with no upgrades or abilities? It doesn't make sense. Of course it is going to be too difficult when you don't have any health and a tiny amount of item slots! Dead Rising's 6-hour story can (should) stretch into dozens of hours of playtime if you're doing it right.
The Halo series tops out at only 5-6 hours per game but shouldn't be considered "too short" because their sandbox-style of design (giving you lots of paths through each level, lots of different weapons to choose from, dynamic A.I., etc.) ensures that each time you go through an area it will be different.
Another example is the Metal Gear Solid series. If you watch all of the cutscenes, the games are 10-15 hours long, but if skip the story there are only 5 hours of actual gameplay in each game. They are 5 hours of dynamic gameplay, however, that can change each time you replay it based on your improving skill level as well as the wealth of weapons and paths and options you have at your disposal in each scenario.
Skill Improvement Is Its Own Reward
The improvements you see in your skill level as you play more of a game should also be considered before you decide a game is too short. You can whip through a story mode in a fighting game in 20 minutes, but you probably aren't very good at it until you play a few hundred more matches and dedicate a lot of practice to it. That doesn't change the fact that you can "beat" it in 20 minutes, but your level of enjoyment that first time through compared to your hundredth time through is likely going to be very different because you'll be better at it.
Shoot-em-ups like Ikaruga or Raiden or Akai Katana are the same way. You can beat them in less than an hour, but you probably aren't playing them the right way, and learning how to play them the best way, and then topping your high scores, stretches that hour of gameplay into dozens of hours. Likewise, you can stumble and fail your way through every level in Trials Evolution in a matter of hours, but going back and re-playing levels and getting better scores once you figure out how to play is the real reward.
It is a very oldschool-style of design to make games like this. Older games on the NES or SNES weren't exactly lengthy (outside of RPGs, of course) and many could be beaten in less than an hour, but no one complained about it back then. That was because they were replayable. They had hidden secret stuff. They had hidden codes. They were challenging, but rewarded your skill when you played them enough to get good at them. Just like Dead Rising, or Lollipop Chainsaw, or Revengeance are today, but instead of playing them properly, gamers these days are more inclined to complain that they are too difficult or too short.
Of Course, Some Games Are Actually Just Short
Some complaints about short games are valid because the games aren't very replayable. Call of Duty or Battlefield 3, for example, are indeed too short because their campaigns are so scripted and linear that they aren't all that replayable. They will be pretty much the same every time. Asura's Wrath, a totally different genre of game, is also considered short and not replayable for the same reason - it is too scripted so everything plays out exactly the same way every time.
This is how we used to play games back in the 80's and 90's. We only got 1-2 new games per year, so we played the heck out of them and explored every nook and cranny to get maximum enjoyment out of them. Gamers today are a bit spoiled by both the variety and availability of games.
The next time you see a game length mentioned in a review is only 5 hours, stop to think for a minute about the context of those 5 hours (which I always try to spell out in my reviews) before you (figuratively) stomp off to your favorite forum to bash the game for being too short. And, hey, while you're at it, maybe you can actually read the text of reviews instead of just getting mad at the score ... Sometimes 5 hours is just 5 hours, of course, and you have every right to complain. More often than not, however, there is additional replay value that will greatly extend the playtime of a game, so put those gameplay hours into context before you decide something is too short.