It's kinda interesting to see how superhero movies evolve. They actually contribute to comics in an interesting way that is rarely talked about. Superman, for example, didn't fly until he was featured in those classic Fleischer cartoons. Superman also was far more humanized in Superman the Movie, than he was in his comics at that point (I even maintain that the movie gave the Man of Steel a Marvel makeover, complete with a very Stan Lee-like Lex Luthor). Alfred was brought back from the dead to be on the much-hated (not by me) 60s TV series, and is now a major part of the Batman mythos.
Years ago, when Teen-Age Mutant Ninja Turtles were made into a movie, I found it a little odd that much of what it was lampooning (mostly Frank Miller's work on Daredevil) was never seen before by most movie-goers, and when Daredevil (and Electra) finally made it to the screen, besides being not-great movies, they seemed liked straight-faced Ninja Turtle films. Now, super-hero teams are being deconstructed before most movie-goers have seen a JLA or Avengers-style super-hero team movie (I guess there have been lots of animated versions on TV though).
What will be for me interesting to watch is, assuming Watchmen is a big success, if the studios will then feel obligated to turn JLA and Avengers into big dark Watchmen-type films. If so, I think that could actually hurt the appeal of those films, and wind down the super-hero movie trend. If, they're able to do a successful upbeat (actually heroic) super-hero team movie, then I believe, the super-heroes will keep on coming.