Markos is hyper-partisan. This means that some policy positions are sacrosanct because they are considered essential to holding the Democratic Party together--examples are opposing Social Security privatization and behaving morally on torture. It also means opposing Moderate Hero-dom and grandstanding attacks on the party for one's own career like Lieberman does.
Beyond that, his mantra is "more and better Democrats," with "more Democrats" being those who are willing to stand by their party label and not undercut the leadership any more than they have to in order to represent their districts. You will never see Kos criticize Walt Minnick or Gene Taylor, for example, unless they vote some way which is truly bizarre from both a partisan and a self-preservation angle. And Kos raised a ton of money for Don Cazayoux and Travis Childers despite the fact we knew they'd be conservative Dems.