Back when I was in college, when we asked a professor how long our papers should be, he answered thusly: "Like a skirt-- short enough to keep it interesting, but long enough to cover the subject." More and more, it seems that you could say the same thing about videogames. In days past, the length of a game was a selling point-- the more gameplay you could get out of it, the more the game was worth. And so MMOs especially ask for a lot of investment, and were asked to give a lot of gameplay in return. I used to beat old NES and Game Boy games in minutes, and now, like Fullbright, I've got days and weeks logged in the MMOs I play.
But is that a good thing? I hate to keep harping on it, but Fullbright brings up Portal for me (a game that seems destined to redefine what we think of as games this year)-- there's a game that only takes a few hours but delivers an experience that lasts much longer than that. In terms of MMOs the current trend seems to be both towards shorter gameplay and less investment for that gameplay.
MMOs may be the exception here-- you sit down to create one character, and heading into a persistent world should make you want to be there. But with many MMOs on the market than before (and an older, busier audience than ever), the trend seems to be headed towards shorter games, not longer. That's not to say that a great game shouldn't cover the subject (no game should end before its time), but designers are feeling more and more pressure to make things short enough to keep it interesting, rather than throwing in random FedEx quests to brag about a few more hours of gameplay.