The main reason I am here: Fallout is the only RPG ever that has held my interest through multiple playthroughs (I am currently on #6), to the point where I can fire it up any time and be totally immersed in it.
I love RPGs, and they are pretty much the only genre of game that I play. I have played lots of them and found many that were excellent and thoroughly enjoyable, but once I played them, that was it. Some games try to manufacture interest in repeated playthroughs by offering "medals" or whatnot for doing this-or-that, or playing through with different races/classes/whatever, or seeing all the different endings, but if the game itself doesn't hold your interest, it feels like more of a chore than something you do for fun.
Thanks to its embracing of user-generated content, plus the fact that the game is thoroughly enjoyable to begin with, it is possible to play over and over and never get bored. (The only caveat being that it would be nice if it weren't so plagued with crashes.) The game can be a very different experience every time. While I look forward with great anticipation to the next release, I can be very happy with Fallout 3 and New Vegas until it gets here.