Obviously workrate and pressure, but his workrate is effective because of his great footwork and balance. His handspeed is also underrated. Specifically, when infighting, he can shift his feet very quickly while staying in balance to land power shots. Lots of guys shift their bodies and move change angles to land effect shots, those shots often lack the full power of a balanced, technically sound shot. Hatton can shift his feet to change the angle of his left hook to the liver as well as anybody I can remember. He also knows how to fight dirty/bully when he needs to go there.