5. Get the tone right.
Some comics books are fun thrill rides. Some are grim angsty nightmares. Some have been both at various times. Many comic book characters have been around for 50 years now and are still in publication or at least appear in other characters' titles, and a few have been around for almost a century. That's a lot of moods to choose from. Pick one and stick with it. Also, the style of special effects matter. Bright flashy ones? Natural effects attached to people? Smooth transformations? The appropriateness of each varies.
Examples: This is one of the few problems I had with the Watchmen movie. It's a superhumanly-badass, holy-shit-that's-awesome movie based on a comic book that's actually a grim, down-to-earth deconstruction of superheroes. If people like this existed, it would suck, they would be completely screwed up, and the whole world would spin out of control. But that's not the impression you get in the movie. The viewer winds up exulting at scenes that should be hopeless, tragic struggles. Fun, sure, but misses the point completely.
And another interesting example is the recent one that inspired this post and the ridiculously long previous one: Thor. The style was unique, I'd say. Its world is not a smooth, high-tech place like Iron Man's, nor is it an organic living environment like the X-Men movies, nor is it the quaint world of Superman movies that forces all the focus back onto the hero. Instead, it really looked like myths. Odin's scenes in Jotunheim could have come out of the Old Testament. Thor beating up SHIELD agents could have been one of the Labors of Hercules. Asgard is not just another planet, it is quite clearly supernatural.
As for the story, it was good. Comparing it to any of the recent Iron Man or Batman movies would be comparing apples to oranges - there's no analogy to current political events in Thor, the hero starts out beating everyone up and has to go through the movie to learn why he shouldn't, only half of it even takes place on Earth - but the story is very much a family drama about a well-meaning asshole who needs to grow up and his passive-aggressive brother with something to prove.
Plus, of course, the whole "mythological" thing.