Part of the comparison between Heath Ledger and Safin is the "old soul" quality. There was this amazing quote in a GQ article years ago where someone said that life isn't as simple for Marat as it is for other players. He's intelligent, he reads, he's interested in art, there's a softness underlying all the racket tossing, life is about far more than tennis, etc. You can picture him sitting with you having tea and discussing Plato or Nietzsche. Sometimes he seems like a wise, time-hardened old man. And, yet, he sometimes seems like a little kid. Heath Ledger, by all accounts, seemed to have the same traits and depth of character. You could certainly never describe either of them as boring.
With McSteamy (a Grey's Anatomy nickname), it's the pure physical resemblance (their height, body build, etc. is very similar along with the facial hair). Plus, McSteamy has . . . ahem . . . a lot of girlfriends on the show