Fun: Some sort of plane travel/planet hopping/extreme environment exploration/"alternative physics" that my character can control and interact with. I don't know how much this overlaps with genre-mashing, because I haven't played most of the games that specialized in this sort of thing (Rifts, Torg, etc.) The game system/setting that I've had the most success with outside of D&D is Shadowrun, and I think that the "astral space" and "virtual hacking" aspects of SR are partially what made it fun and interesting (also see below). (Fantasy-literature-wise, I'm coming at gaming from a Zelazny/Farmer direction rather than Tolkien/Jordan.)
Fun: Being able to play beings which differ substantially from the human(oid) norm in a game-mechanical way (i.e. that aren't just "humans in funny makeup" with different stat bonuses and minor powers). D&D Dark Sun did this to a reasonable degree, as did Shadowrun. Core D&D races aren't reallly "different" enough. (Needless to say, I was a huge fan of Savage Species and the cleaned-up LA/ECL rules. "I don't care how much you nerf me; just let me play a damn blink dog/lantern archon/whatever.")
Fun: Being able to play beings which differ substantially from the human(oid) norm in a game-mechanical way (i.e. that aren't just "humans in funny makeup" with different stat bonuses and minor powers). D&D Dark Sun did this to a reasonable degree, as did Shadowrun. Core D&D races aren't reallly "different" enough. (Needless to say, I was a huge fan of Savage Species and the cleaned-up LA/ECL rules. "I don't care how much you nerf me; just let me play a damn blink dog/lantern archon/whatever.")