IMDB Doesn't Like Black Films
They don't. There is no reason why Roll Bounce should be 4.5. Definitely 5.5-6.5, but 4.5 is ridiculous. I realize it's an ethnic movie, but the story is perfectly decent, if a little cliched, and the roller skating, I mean, c'mon, is just cool. RT clocked it at 66%, and c'mon, it's not exactly like it's hard to watch.
Maybe the majority of IMDb users are white, or something. Which isn't to say that IMDb users are racist. More to say, their opinions of films are disproporionately skewed.