I think one of the problems with superhero films is that even the best of them have a bit of - forgive me - campiness in them. It is hard to take some of them very seriously given some of the costumes and trappings and props.
Batman is one of the better franchises in that he is just a normal man (ie no actual super powers) with cool gadgets and some training in martial arts.
But even then, some of the older Batman films are memorably awful (anyone up to watching Batman and Robin? Didn't think so. 
)The first two Spiderman movies were pretty good in that even with his super powers, the main character was still just a geeky kid who had trouble getting dates with his dreamgirl.
The farther the characters get away from being a regular human, the greater the chance of campiness, the harder it is for audiences to identify with the characters, and the more a moral or ethical lesson might get lost...