Your waaay late to the Dane Cook hate train.
For a long time there were rumors that he stole jokes... specifically a couple from Louis CK. He was actually on Louie (Louis CK's TV show) a little while ago and they talked about it (though, to be fair, Louie is a sitcom not a talk show or anything like that). Otherwise, the only reason he's so "hated on the internet" is because he was very popular. Internet nerds hate to go along with the rest of popular society, less they be compared to a "frat guy" or a "college douche". He's also considered relatively good looking and broke into a couple of small acting roles.
He was never a favorite of mine but his stand-up never bothered me, I remember laughing pretty hard at some of his old ones on TV. The last one I saw was that HBO special he did, where he stood on that big circle stage, I thought it was funny.