Liberals love to think of themselves as intellectual and nuanced, but liberalism is incredibly simplistic. It's nothing more than "childlike emotionalism applied to adult issues." Very seldom does any issue that doesn't involve pandering to their supporters boil down at its core level to more than feeling "nice" or "mean" to liberals. This makes liberals ill equipped to deal with complex issues.
Since liberals tend to support or oppose policies based on how those policies make them feel about themselves, they do very little intellectual examination of whether the policies they advocate work or not. That's because it doesn't matter to them whether the policy is effective or not; it matters whether advocating the policy makes them feel "good" or "bad," "compassionate" or "stingy," "nice" or "mean."
Because of this, liberalism has more in common with religion than it does with other political ideologies like conservatism or libertarianism. Moreover, liberal beliefs are more like religious doctrine than any sort of battle-tested policies that bear up under logic or examination. Although the interpretation of the doctrine that the Left supports may change a bit over time, just as religious doctrine does, it's essentially taken on faith, like scripture.
That's why, for example, you may see ferocious debates on the right side of the blogosphere about the war, illegal immigration, or spending. But, with the netroots, the debates almost always revolve around the best strategy to get more liberals elected. The issues are not really up for debate, other than debate over how to get them enacted.
This same thinking leads to very little criticism of liberals by other liberals. Liberals will ferociously defend and even happily echo the lies of other liberals. Liberal feminists will defend Bill Clinton and Ted Kennedy. Liberals who pride themselves on being tolerant of other races will support Robert Byrd. Why? Because even if they're wrong, they're still fellow liberals -- which must mean they’re nice people. What this leads to is an attitude that can be summed up like so: "The only things that a liberal can do wrong is to be insufficiently liberal, to question an important plank of the liberal agenda, or to do something politically that aids conservatives."