I enjoy games that create a challenge for the players that they must overcome in a variety of ways. Not every challenge can be resolved with force, and sometimes subtlety is needed in the form of verbal sparing, misdirection, confusion, intimidation or sheer intelligence. I fully believe that the history of a character makes them what they are at the present. I like back stories and how they play into games.
I have been playing RPGs for close to 20 years and have tried several different games. I have always ended up going back to D&D cause I enjoy the challenges of doing things in that environment.