His passion and excitability. I turned away in loathing when Sullivan, long after even the most partisan hacks had given up on the execrable "Trig Palin is a changeling" legend, continued to promote that conspiracy theory. I laugh at people who say "Andrew Sullivan is my favorite conservative blogger," because Sullivan, befitting his passion and excitability, abandoned conservatism entirely when George W. Bush cynically endorsed an amendment to the Constitution prohibiting same sex marriage in 2003. (Sullivan was right and Bush was wrong.)
But for a leftist, which is what Sullivan is now, his stance on individual freedom remains excellent, and that's nowhere better reflected than in his coverage of the (hopefully) emerging revolution in Iran. For the past 4 days, Sullivan's site has been the best source for the sort of news that isn't covered by mainstream press, individual twitter messages and underground video of protests and massacres. And if his passion infects his blogging (Sullivan really believes that a second revolution may be at hand), well, it wouldn't be Andrew Sullivan without the passion.
Sullivan's major league blog doesn't need an endorsement from a AA (I flatter myself we're not single A) blog like Popehat, but I urge you to read it anyway.