In terms of price, I have a theory that games would sell a lot more and a lot better if they were all priced around $25-$30, more comparable with film DVD's. So far, however, the only thing I have to prove this is Nintendo DS software sales. A lot of experimental titles are being purchased for that system, and I know before I got rid of mine I usually thought "$30? Meh, even if it's not great, it's not like I spent a fortune". I ponder if more games would sell if they were made cheaper, even if the overall profits remained same.
But that's just sort of a theory and has nothing to do with game length.
I look at it this way. 12-15 hours of gameplay is decent enough, as it means I beat the game in about a week. If it's on Xbox 360, I can then spend extra time trying to score achievements, replaying it on a harder difficulty, etc. These can add around 5 to 29342038420394 hours, depending on the game (nailing the Zombie Genocider achievement in Dead Rising took quite a while, yet, oddly enough, was still entertaining). On a Wii, it simply varies from game to game. After beating RE4 on the Wii, I began to replay it until I had to start my summer co-op and also had other games coming out (like Overlord). RE4 nets about 15-20 hours itself, and still offers replay. Metroid Prime 3, on the other hand, while it offers a lot of hours initially, I had no reason to go back and replay. Even on a harder difficulty, as my experience with the previous Prime titles pretty much have convinced me that hard mode = more tedious mode.
So, MP:3, a 20 or more hour game, hasn't gotten the same overall dedication that Dead Rising or RE4 have gotten. Hell, I've already netted more time into Halo 3, and I haven't even played multiplayer outside of co-op in that.
I guess my summary here is that I could care less if the game is 12-15 hours as long as I have reason to go back and play it more. Of course, Halo 2&3 and Gears of War all resulted in one similar feeling: there needed to be at LEAST one more level. For Gears and Halo 3, I think it's mostly from a lack of satisfaction with the story. Nonetheless, even though the first play through was short, I've still enjoyed replaying those titles numerous times.
Meanwhile, I still haven't beaten Zelda: Twilight Princess, simply because so many games have come out since it has and, honestly, 40+ hours is just too damn long for me at this point in my life. If I can't beat it in a week, then there's a good chance I may never beat it at all.