as others have said, Dane is hilarious because of how he delivers the jokes. I've been a fan for quite a while, picked up his stuff starting at his second cd and then went to his first cd. Loved the HBO special, because now I actually got to see the energy. That was the problem with listening to his cds... you could tell he was doing some kind of action on stage by the way he would talk through the joke, but without seeing that energy it felt like it was only half the joke, but yet I still found the material very funny.
I'm no comedy connoisseur, but I've enjoyed quite a few HBO comedy specials and some of the shows on Comedy Central, like Lewis Black's show.
the problem with Dane's material though, is it is easily quotable because anyone who has heard his stuff can hear a quote and pick up on the context of the entire joke. So, a quote by itself may not be an entire joke, but the context ends up being hilarious. With that in mind, his material gets worn down very easily among the college age crowd. So many people find it necessary to use quotes of his material in every day life. My best friend's girlfriend is that way and it's extremely annoying. It was funny the first couple of times, but since your not Dane Cook himself, the 24th time you've said it has made you look like an ass. Stop. Now.
But that tends to make the jokes, when presented by Cook himself, even better when I go back and watch or hear his material again, because now it's coming from him.