Everyone is raving about Mickey Rourke's performance, and I have nothing new to add (except maybe to point out that if you watch enough indie films, this wasn't a comeback at all. He's been around the whole time).
Instead let me talk about Darren Aronofsky. In a way, after the terrible box office of The Fountain (which I loved) he was more in need of a hit than Rourke. Off the top of my head I can't think of an auteur who has more command of the visual language of film without having a distinct visual style. We can talk about a film having a Soderbergh look, or a Scorsese look, or a David Fincher look, etc. If you didn't read the credits, you might not guess that the same director made Pi, Requiem for a Dream, The Fountain, and now The Wrestler. Every time out, it's like he reinvents a whole new visual language, borrowing heavily from the history of film but not necessarily his own filmography.
In The Wrestler the visuals are all about heaping pain and humiliation on Rourke's Randy "The Ram" Robinson (nee Robin Ramsinsky). Just watch how many scenes follow Ram from behind, the camera's eye resting on his back like it's the weight of the world. That has just as much to do with the camera work as Rourke's ability to walk around with stooped shoulders. Look how his boss at his menial grocery store job is physically placed above him--on ladders or steps--so he can literally talk down to him. Ironically, the uplifting moments are when he's covered in blood after a match. That's his world, and that's where he thrives.
That's not to forgive Aronofsky for being one of the least subtle filmmakers out there. A scene where his stripper friend Cassidy/Pam (Marisa Tomei) compares him to Jesus in Passion of the Christ is a little overblown. The irony is that Ram does nothing to redeem himself or anyone else--his only similarity is his ability to take gobs and gobs of punishment. Does this make it a particularly anti-Christian, anti-spiritual movie, by reducing Jesus to the level of a beat-up steroid junkie deadbeat dad? Maybe, or maybe it's neither that clever nor that provocative.
If there is one consistency across Aronofsky's film, it's his penchant for parallel action. That's a story structure element--not a visual element--that he likes to use, so my previous statement that he's a visual language master with no distinctive visual style still holds. In The Wrestler the parallel action is between Ram/Robin and Cassidy/Pam. Each holds a stage name and a real name. Their names even sound nearly the same--Ram/Pam, Ramsinsky/Cassidy. Each gets up in front of people as a physical specimen for the audiences visceral enjoyment. In fact, the only time Ram is alive outside the ring is when he's dancing--a physical motion that bridges the gap between wrestling and stripping. The difference is that while Ram avoids his real life (he refuses to even let anyone call him Robin), Pam is only using her stage persona of Cassidy to create a real life for herself. She has a son, she's planning to stop stripping and move to a condo in a nice neighborhood with good schools. Even the parallel names show that--Ram is Rourke's character's stage name, Pam is Tomei's character's real name, and both are the names they'd rather go by.
So yeah, The Wrestler is a great movie, and Rourke fully deserves the credit he's getting. But its greatness goes far beyond him, to the entire cast and to director Darren Aronofsky.