Casting Sam Worthington over literally everyone else in Hollywood to save money. His American accent is distracting and he can't act. He plays the same character in every movie like Seth Rogan. I don't get it.
Nonsense. The studio offered Cameron pretty much every leading man in Hollywood and he "begged for" Sam Worthington because he made him laugh at the first audition or something. Any leading man would have taken this role just because of the budget and Cameron being the director. Anyone, they could have even gotten Ledger to do this at the same time as The Dark Knight I remember reading. It was filmed during the same time period, but the live-action dates didn't conflict with The Dark Knight and Cameron was set on Worthington. Worthington later said it was because he was cheap, when Cameron had a quarter billion budget.
To be fair, a lot of actors would have tried to make that character seem to be anything other than the slow-witted grunt he was intended to be. Worthington at least resisted the temptation to make him the sort of brooding antihero or a neurotic mess that most actor think is "deep" and gave Cameron the slow-witted grunt he wanted.
Whether that was by design or natural slow wits I'll never know, but if Cameron wanted a protagonist who wouldn't take too much attention away from the FX, that's what he got from Worthington.