he is just very hard to beat.
pretty much everyone has already said most of this, but he has a steel jaw, great sprawl-really good at stopping guys from taking him down, throws punches/can deliver knockout power from very odd angles-one of the few people that can knock you out when backing up, awesome kicks,remenber when he knocked out babalu!!! omg that was awsome. that would have been like getting hit with a baseball bat in the head.
the only person i have ever seen him fight that his style just didnt "work" against, was keith jardine, funnily enough. keith just took him out of his game by working the leg kicks, making chuck come after him and countering the counter fighter. big upset, but nicely worked by keith.