One underplayed aspect of this year's elections that I think could make an impact is the return of disaffected Republicans who didn't even bother coming out to vote in 2008 because they were unhappy about the nomination of John McCain. They may have felt it didn't make any difference who they voted for then, but feel differently about things now that they've seen the impact of Democratic control in Washington.
On the Virginia exit poll 94% of respondents said who they voted for President last year and their responses average out to a 13.2% advantage for Bob McDonnell. Since McDonnell won by 17 points that means he won the votes of non-2008 voters- either new registrants or more likely folks who just decided not to turn out in '08- by a margin somewhere in the neighborhood of 80-20.
That fits in with what we're finding on a lot of our polling. Our last national survey found Barack Obama's approval rating with people who didn't report a 2008 vote at just 13% and gave Republicans a 42-15 lead on the generic Congressional ballot with them. We've found similar results on some of our race specific 2010 polls- a 56-15 lead for Mike Castle over Beau Biden, a 62-16 advantage for Michele Bachmann over Tarryl Clark, and a 40-27 one for Scott Walker over Tom Barrett.
We don't really know how many cranky Republicans just stayed at home for the last Presidential election but there are some preliminary indicators that they could account for 3-4% of the electorate this year and that's enough to make a big impact in some of the closer races.