Per much of what I've read and seen, Obama won by holding his own with Clinton. He seemed Presidential. She's hit her ceiling (check out the Frank Luntz focus group in the link above) and he's still finding his. They said he was a great closer, but no one has expected this. Crowds like this for a primary speech?
There's also been this brewing scandal for Clinton, mainly another reminder that we don't need her husband distracting us for the next four years. And there's been this new truly key endorsement for Obama.
Most of all, the debate was a rebuke to the literally empty Republican party. Empty of ideas, empty rhetoric only appealing to their shrinking base, empty of anything approaching a worthwhile candidate for November.
There's even rumor of Mitt and McCain both dropping out...