For all that I've been playing for 7 or so years now, I consider my experience fairly limited, because except for 1 abortive campaign (as in, we only did 1 game of it), I've only played with 1 group. The membership's changed a bit, but the core people are the same, and 9 times out of 10, it's my brother who's running it. Within that context though, I've played D&D 2 and 3, Vampire, some silly Marvel game years and years ago, Aberrant, and I'll be playing in a Star Wars game in a couple of months. I made up some Changeling characters once, but never got to use them. Like I said, limited.
D&D gets a lot of bad press, but it's no worse than any other game as far as I can see - it's all in how it's run. If the DM and the players just want to do hack n slash and dungeon crawls, then it will suck. But it's possible to play Vampire doing nothing but killing. If you get players who are willing to put effort into their characters, and a DM who's willing to develop an interesting setting and a compelling storyline, then you can have a D&D game as engrossing as any other.
I'm at a point now where I really want to play with other groups, if only to see if I'm as good a role-player as I hope I might be. I tend to get really wrapped up in creating characters, with lots of background and everything, but the games my brother runs tend to be very complexly plotted, so we spend so much time trying to figure everything out that we don't interact in character as much as I would like. But maybe that's just my excuse, and I'm not actually that good regardless.
So tell me - what do you prefer? Is it possible to integrate really complex characters with really involved plots? If you had to sacrifice one, which would you give up? How in-character are you able to get? A friend of mine once cried when her character's mentor got killed.