Once again, I refer readers to Professor Jacobson's roundup of the Best Tweets of the Debate.
Beyond that, I offer my own quick impressions: Romney played it conservatively tonight. His goal seemed to be to distinguish himself more from Bush than from Obama. Personally, I wish that he drew a few more distinctions between himself and Obama, particularly on Libya. Nevertheless, he came across as presidential. Someone the American people should be able to trust as commander-in-chief. That's the most important takeaway from the night.
President Obama, by contrast, came across as someone trying to play catch up. He tried to be aggressive, but those efforts mostly came across as petty and snarky. Not qualities most people look for in a president.
On that score, I think Romney clear won the night. More importantly, Romney has had the momentum ever since the first debate and he did nothing tonight to lose it.