I've read in several places (and feel the same) that Windu's character was just not exciting enough. Even though he was an incredibly skilled Jedi who could defeat Palpatine and served in a leading position, he barely has any memorable lines or actions outside his final climactic duel. There's so much potential that just went to waste.
Yeah, it was poor David Prowse's Bristol accent which led to the casting of James Earl Jones as the voice of Darth Vader - something Prowse wasn't informed of until the film was released. He was understandably salty afterwards, and this led to the row with Lucas.