Well, physically, like most people have said, he's sort of Cap with a rifle and sword, but a lot faster, a bit more agile, and smarter. And he has Wolverine level healing also. He's smacked around Batman twice, worked over the JLA recently, and nearly embarrassed the X-Men in their Teen Titans crossover, one of being Wolverine by hitting him with an explosive blast to the chest (I know most crossovers hold little water, but some of the things he did there are still pretty believable). Deadpool's regeneration is hard as hell to overcome, regardless Slade could still school him on the art of whoopass. If you need to learn more about Deathstroke, this should help a bit: