For one, he's remarkably intelligent-sounding in this clip, despite the fact that he's rambling a bit and talking about taking psychedelic drugs (Warning! - plenty of profanity here!):
The second mind-changer? He slams Carlos Mencia and gets a bunch of comedians to get the courage to call him the joke-stealer that he is. The video's appearing and disappearing from Youtube, at times stating it was removed for copyright infringement. So if the video below doesn't work, try the Myspace link to the video instead.
Update (02-16-2007): According to Joe Rogan's website (Warning! - plenty of profanity here!), he has been banned from the Comedy Store and his agent (who is also Carlos's agent) has dropped him as a result of the whole incident in the video above.