Early on, I felt that d20 could handle the really high-end type of superhero - Superman, the Hulk, Thor, Martian Manhunter, Green Lantern, and Supreme being good examples of that ilk. Now, I'm thinking that it'd be hard for d20 to handle them without major revisions. I hope I'm wrong.
I do think that d20 can handle many of what I fondly refer to as the "second-string heroes." These are the heroes that have more, for lack of a better term, "down-to-earth" powers, if they have powers. Dr. Strange, Captain America, Spiderman, and Batman are good examples. It's not a disparagement of them, and many of my favorites are from their ranks. It's just that their powers/abilities are more easily modelled by d20.