I'm surprised so many people don't like his comedy. I'm a huge stand up fan but I'm not a big fan of his stuff either.
He sort of tries to be like George Carlin at the end of his career where he wants to talk mainly politics with jokes sprinkled in between. Carlin came off as a master while Rogan comes off as that annoying stoner in high school who wasted all his brain cells on information about marijuana studies and legalization. People go to a stand up show to laugh, not have your opinion forced on to them with a joke thrown in every 5 minutes.
Rogan is very quick witted and is great at dealing with hecklers and going off the cuff on the Q&A bits, but his pre written stand up is pretty bad. He's still very popular because he's on tv, he attracts the 420 crowd, and he can deliver jokes very well even if they are poorly written.