So, with the Mark of Cain, Dean does have some enhanced abilities. He is stronger, and faster, he has better endurance and senses. It isn't really stated explicitly, but it is heavily implied in the way Dean hunts and acts. He also has a little bit of precognition (ie: when he had that vision of the massacre before it happened), and mentions several times having dark visions. He also heals quicker than normal (although not instantly like as a demon), and, is virtually immortal. He doesn't ever really get explicit powers, like with Sam on the demon blood, except as a demon, but he does get enough to make him a better, more efficient killer.
Also, unrelated to that, but I just love the fact that Jensen accidentally stabs Jared with the blade in that scene where Dean collapses into Sam's arms. Because it wasn't supposed to be a stunt, they had been using the metal prop that is used for closeups, instead of the rubber prop used for combat scenes, and apparently went in pretty deep. That horrified look on Jensen's face isn't him acting at all for Dean, it's Jensen in shock and wondering why no one is calling cut to check on Jared, but apparently Jared and Jensen both loved how it turned out, so they kept that take in the show