Let's see - voter turnout was roughly what it was four years ago. Part of the explanation is obvious - enthused Dems were offset by discouraged Reps - and part of the explanation is less obvious:
Several large states, including California and New York, had no statewide races and virtually no advertising or get-out-the-vote efforts by either presidential campaign.
Add to the turnout stats the fact that anti-gay marriage bills passed in three states, including California, and the evidence of a new and enduring progressive majority becomes pretty slim. Two troubling wars and a disastrous economy vault the Dems to a full 52% in the Presidential election - this is a mountain conservatives can climb.
And I will assert, based on hints such as the Rahm Emanuel appointment, that Obama and Company are well aware of their tenuous hold on power. The House incudes plenty of new swing seat Dems who replaced Republicans and could in turn be replaced by Republicans if Obama and Pelosi stagger too far to the left.