He plays the same guy in every movie - badass, scruffy British guy with a deep voice and five o'clock shadow who has to kill somebody. In the majority of his movies, he is bald, but one exception is Guy Ritchie's crap Revolver.
There's some Stathamology for you.
He DOES play the same character over and over in most of his movies, but you say that like it's a bad thing.
He plays a VERY specific kind of scruffy bald bad-***. He plays the kind that couldn't be bothered, and just isn't interested in being bad ***. He's got that peculiar kind of curmudgeonly demeanor that says, "Look I haven't got time for this."
To differentiate, let me describe different bad asses, as defined by the height of their careers.
Keifer Sutherland as Jack Bauer = I'm bad-*** because I torture people and suppress my feelings and compromise my sanity by doing so... and I also cry in my SUV.
Pre-Blade Wesley Snipes = I'm bad-asser than you. Not the most bad-*** person there is, but no matter how bad-*** you might be, I'm just one notch more. I'm like Milton Berle in a dick measuring contest... I only show you the inches necessary to win.
Mel Gibson, Mad Max Era = I'm only bad *** because I have to be. I'd really rather be playing with my golden retriever and drinking iced tea in the middle of the summer, but aw ****, I have to get some gas for my car.
Jason Statham, The Crank Transporter Who Must Stop The One=Please not the suit. Or the car. Look, I'm having a bad day. I'm not bad ***. Leave me alone. He's only bad *** because crazy Asian women won't leave him alone, girlfriends won't listen to him, and people won't grant him simple requests.