The Message
As Politico points out, most people polled say that they weren't voting specifically for or against the President. Fair enough. But before anyone reads too much into that, consider the fact that it's still a bit "politically incorrect" to be "against" Barack Obama; that's why everyone still insists to pollsters they like him and that their vote isn't about him, even as they vote against politicians who share his agenda.
Finally, consider the magnitude of the vote shifts between last year and this. The 20-point swing in New Jersey and Bob McDonnell's comfortable victory in Virginia -- just four years after barely squeaking in -- has a lot to say about the reasons for Obama's victory last year. And they have nothing to do with a new left tilt in the US.
First, there were a lot of people who were tired of George Bush, ready for a change, and disgruntled by the war. Second, John McCain was a lackluster candidate, who didn't have the enthusiastic support of his own party's base. Third, lots of people (especially the young) were interested in "making history" -- without much consideration of Obama's ideology. And that latter factor worked perfectly for the President -- whether by design or because he simply lacked details (or both), he kept his pitch largely agenda- and ideology-free, and the press let him get away with it.
Now the pendulum is swinging the other way. Nervous Democrat congressmen must be realizing that a fuzzy fog of Obamania can't save them from their voters if they ignore the electorate's wishes. And that's as it should be.