Define "bad actor"
In my mind, there is a huge difference between a bad actor and a good actor, but there is little to no difference between a good actor and a great actor. The difference between a good actor and a great is almost exclusively related to the material he has to perform or the director that is making more of the stylistic decisions.
A bad actor however is bad regardless of anything. He/she either can't move naturally or speak naturally and often times seem as if they were a 7 year old trying to recite a book report that they were forced to commit to memory. Bad actors are cringe-inducing in how much they take a viewer out of the movie and back to horibble horrible reality.
So what's my point? I just don't get how people here can think so poorly of Vin Diesel's acting. He is a fine and competent actor which has never once crashed my suspension of disbelief. Certainly, some of the movies he's been in have had horribly unnatural dialogue or inane plot devices, but I can hardly blame that on Vin Diesel's acting ability any more than I could blame a quarterback for sucking if his receivers had no arms.
P.S. For all the people claiming others are jealous of Vin...what are you, nuts? Shouldn't everyone be jealous of Vin? Hell, I'm jealous of all actors making 20 million dollars a flic. Mmmmmmm, 20 million dollars.