While I agree that escapism is a common appeal of RPGs, I don't think escape from reality happens to the extent that many seem to believe.
Knowledge and potential
Any role can be played only so well as it is understood. That's why actors do research. The actor tries to imagine what it would be like to be that other person, and then he acts the part as much as that character's personality happens to overlap with the actor's own personal capacity.
All personalities are far broader than can be witnessed in any single moment. Each personality includes the choices one makes and the choices one is capable of making. That's why good fiction writers know much more about their characters than the reader is shown. And that's why your college professor sees a different "you" than your mom, and her conception is different than your co-worker's, which is different than your boss's, and so on.
Different people and settings bring out different parts of your personality, and nobody will ever know you in entirety. That doesn't make you deceitful. Different individuals need and want different things from us. They have different senses of humor, so you tell different jokes. They have different problems, so you offer different kinds of advice and sympathy. One likes vulgarity; another likes pithy remarks. One person evokes your anger; another makes you laugh. No person will ever see every part of you because no person will ever witness you in every situation; and, on top of that, you're always changing and growing.
The choices involved in roleplay express real aspects of the player's personality. The actions my character performs are actions within the scope of my personality under the same set of conditions. And that last part's the kicker.
Conditions of personality
The only true escape comes when something about the setting creates parameters that are different from the real world.
In real life, many consider me a overly nice guy. I never lose my temper, never hurt someone's feelings intentionally (not even in retaliation), and I will go out of my way to help complete strangers -- all just because it's natural for me to care about every person I see face-to-face. But as a player in Neverwinter Nights, I didn't really care about any of the other characters. So I actually levelled up my character before even leaving town by brutally slaughtering every NPC who was of no use to me. Once, I even cheated an NPC out of his money, and then killed him. I was disappointed when the game wouldn't allow me to slaughter the child NPCs the same way.
The point is... Under this different set of parameters which the game presented (if not intentionally...Bioware probably wanted me to care about their NPCs), I was neutral evil. Under real parameters, I'm neutral good. Actual choices are far more important than potential in defining character. However, potentiality is a part of personality. The fact that I thought to slaughter all those NPCs while a friend who also had the game never killed an NPC needlessly demonstrates a difference between our personalities (perhaps only that it takes more for me to empathize with game characters than for my friend to). Even though those parameters are completely virtual, completely impossible in real life, they reveal a difference between what defines Aaron and what defines Jay.
Few player characters, if any, reflect nothing of the player's personality. And I'll go so far as to say that most player characters reveal quite a bit. What we're escaping from is pressures upon our personalities, not really from who we are.
Every artist is a psychologist in disguise. =)