@Thomas,
Yeah, must've been quite a while since you paid attention to him.
I started paying attention in 2006-2007 when I'd just Google "Barack Obama" now and then, and he (Sullivan) kept turning up.
One thing I really like about him is that he is unusually willing to change his mind in the face of evidence (or persuasive arguments).
So, he's thoroughly repudiated his support for the Iraq war, for example.
He's still hot-headed and sometimes leaps to the wrong idea before coming around, but he seems to usually come around, and his blog is a good clearinghouse of political info.
I don't care what he thinks, particularly, in general -- I fairly frequently send him dissenting emails (which he occasionally publishes -- he's posted my emails maybe 5-10 times, but not all of them were dissenting.) But I find his blog a good starting point for general political info, and I do agree with him (mostly) re: gay marriage. (The "mostly" is that he's a bit more revolutionary about it all than I am -- I'm fine with incrementalism as long as it gets the job done in the end. Especially if the incrementalism is likely to work BETTER overall than being revolutionary.)