There was no sense of drama or that Maximus would die early or was in any real danger. And he was almost like a superhero.
I think when you have a trained Roman war hero fighting slaves and criminals he would look pretty unstoppable
The friendship plot was not developed enough with the fellow slaves and his becoming their leader was poorly done. There was no one to challenge him as the leader, he is just made it.
It was a long movie, they developed their friendship pretty well IMO, and when you're fighting for your life every day your going to grow close to the guys you're fighting a long side with
As far as him becoming their leader, he was a leader of the Roman military, and easily the best fighter in the group; if they wanted to live it was in their best interest to follow him, makes perfect sense to me