I enjoy game-worlds that award experience points both through battling enemies and solving puzzles. I especially appreciate it when the GM incorporates player ideas into his/her campaign. I also believe the best GM's are versatile with their game and allow the players' actions to unfold the story rather than railroading their characters on a single predetermined path.
I have mostly played AD&D with my friends, and occasionally, some Vampire: Dark Ages. Because most of my friends are long distance, I've had to role play via Skype as of late. I am a full-time student during the fall and spring semesters, so I may not be able to play more than a couple times per month during those times. However, my summer schedule is much more open, and I could potentially play a few times a week.