Heavenly Sword, Bioshock and rumored COD 4 to be short. Heavenly Sword was short but it certainly was immersive, the voice acting, the storyline were all awesome. Bioshock, I haven't played it but based on the reviews it is probably very immersive as well. COD4 if it is only a 6 hour play through it has a multiplayer which makes up for that.
Halo 3 had multiplayer, don't know how long the single player was.
Ratchet & Clank, 10-12 hours ...isn't that pretty long?
Your also not factoring in the replay values, in every ps3, xbox 360 game every level is completely different, the environment is completely different, the sounds, etc. I'm not talking about just drawing a few boxes into the same screen and mixing up where some guns are placed where after you play through the game once you would rather not do it again, but levels that just make you wish you could rewind and just play through that level again.