I'd also add that it works pretty much for any adaptation, not just comic books, obviously. Kinda helps that I've rarely ever seen some adaptation after source material, and I must say that this way movie can't rely on the fact it's about someone's favorite character and whatnot. Gotta stand on its own legs, as you've said, and the problem is that pretty much superhero format itself makes works quite shaky, especially if the basis - the human story, so to say - is inevitable sacrificed to allow more action scenes and other "awesome stuff" and whatnot.
I guess that a single movie must try freaking hard to achieve everything within its time and other constraints. It's up to us to decide whether that means we should accept what we get for the best it was possible to do or decide that it's just a recipe for a problem and move on.