Recently, there have been some complaints in the Wii forum that five to six hours of gameplay is too short for a $50-game. These complaints are based on the reviews of Deadly Creatures, which generally place the game at less than ten hours, and previews of MadWorld, specifically at IGN, which places the game at five to six hours.
These time estimates don't take replayability into account, in the sense of exploration, unlockables, multiplayer, and even simply playing through the game multiple times. This is nothing new, but people seem to think that the time it takes from the start of the game to the credits is the maximum play time we can get out of a game, when that's (pretty clearly) the minimum.
So please, consider these things before getting discouraged and letting everyone know about it when other people say that a game isn't as long as you expected.