Cap's a very VERY enjoyable film. Having seen it twice, I can tell it's one of those movies that you could watch a hundred times and still want to see it again.
But it's missing something. A certain level of depth and emotional impact that pushes it over that hump from good movie to great movie. A movie that you not only love watching, but will be forever remembered as a piece of great cinema.
I think TDK does that. Partially because of Ledger's performance. And that's not really an insult to TDK, cause, if you took Daniel Day Louis out of There Will Be Blood, would it be any good? Or what if you took De Niro out of Taxi Driver, or George C. Scott out of Patton? Actors make certain movies. And there's nothing wrong with that.
But the real key heat makes The Dark Knight more praised then other superhero movies is...it's a movie in and of itself. It's not a superhero movie. It's a gritty, crime drama that happens to involve comic book characters. You could take Batman, Joker, Two-Face, etc. out of it, and as long as you have those archetypical characters and that plot, and that stunning performance by Ledger, it would be a great film.
Captain America, however, is about Cap. If you took him out of it, you wouldn't have a movie. At least, not a compelling one. And there's nothing wrong with that either; I love that Captain America is such a wonderful testament to the character. It's great. I have no complaints.
But to say it's better than TDK is just highly inaccurate in my opinion. And that's not as a Batman fan, or a comic book fan, that's purely as a lover of films.