Picking one thing is difficult. The social side is wonderful and the nostalgia of playing the same general game I did when I was twelve is also fun.
But for me it is the chance to exercise this muscle called an imagination. Too often in our boring lives filling our roles in society we have to be stoic, reasonable, responsible, and focused on boring small insignificant tasks. I enjoy the opportunity to be whimsical, fantastic, and surreal. I enjoy the fact that for a few hours a week I don't give a damn about balancing checkbooks, presidential elections, or filing out the work expense report, but rather slaying evil orcs, battling extraplanar excursions, or just saving the damsel in distress.