Comic books are about outlandish, amazing things happening, and how a wildly diverse and interesting cast of characters respond to those things. For that reason, more than most other genres, the GM should be prepared to say "yes" to whatever craziness the players intend to try, as long as it's fun. The PCs will be heroic, larger-than-life sorts, and can be expected to do impossible things.
Sometimes this dramatic license can go too far, or take the game in undesirable directions. If your goal is to emulate the style of 80's Marvel comics, and some of your players are more interested in 90's Image, getting on the same page before the campaign begins is essential.
When finding your voice as GM, it's helpful to think of yourself as the narrator, rather than the opposition. Your role is to introduce the PCs, help tell their story to the audience, and put a spotlight on their strengths and victories. You can be a fan of the characters and still throw a city-wrecking giant monster at them.