I get to... Well. It's not something easily put into words.
I could say that I get to live an entirely different life - existing in a world that lives and breathes that is so far from the dreariness of every day, where status is less important by far than my own merits. It's not where I'm coming from, but who I am and where I'm going that matters. These people that I act through... They're so much different from me, and yet, so similar.
I could say that I get to explore - both outwards, reaching for the most obscure and unheard of nooks where none has laid eye or foot, and inwards, seeing new sides to myself that I never knew were there. Through these characters, these people, I get to experiment with just who I want to be, getting closer to my ideals, and even take steps further away from myself(!) by playing these people with whom I have less in common.
I could say that it's the interaction - knowing people, both in the community and in the gameworld, and seeing the dynamics between them. How one person's real-life experience can influence the life of a character, or how a character's development can teach very real lessons to a player... How these people, though fictional, take on lives that are so much their own, so surreally complex and dynamic with the world around them.
I could say it's achievement - gaining in levels, wealth, conquering evil in iconic ways that so few will ever experience in "real life." Gaining, however vicariously through these fictional characters, a sense of capability and confidence in myself, and growing through them into a more rounded, charismatic person. Gaining recognition amongst my peers in the community for my accomplishments and deeds of derring-do, or notoriety for my nefarious plots...
I could say a lot of things.