When I was developing Heroes and Villains MU the initial idea was much broader and was going to be powered by FATE Accelerated. The idea was a game where nearly any multi-media character could exist. It was a sort of post-apocalyptic fantasy game, Rifts meets Thundarr sort of thing. I played in a tabletop game that used the same theme and FATE Accelerated worked wonderfully for it.
Well, during the building phase I had actual conversations that went like this...
Player: I'm planning on playing Hulk Hogan who has a +4 in Forceful just like Superman! So that means I can smack people with cars, right?
Me: No, Superman has a narrative reason to be able to throw cars that takes the form of his Last son of Krypton aspect. Hulk Hogan doesn't have anything on his sheet that shows he should be able to throw cars. But, narratively, his primary focus is his strength so he has the same impact as Superman when using it he just has to use it in different ways.
Player: Uh, they're both +4 in Forceful so they're just as strong as each other. I don't see why, if Superman can throw cars, Hulk Hogan can't either.
So I revised the theme and decided to use a system where limits and levels needed to be defined. I've found that when they aren't defined people will literally do whatever they want and then hold up the game/scene/whatever as they argue about how they're right no matter what you or other s say.