This is the interesting heart of the matter. As we've heard time and time again in the lead up, the crux of the modern debate is a contest of presentational style -- an ability to engage your opponent, engage the moderator, and most of all, engage the audience. It's not about facts or plans, so much as it's about inspiring confidence in the electorate that you've got plans under your belt, and you've got accurate facts well-filed in your skull. That's presentational.
I'd watched the 2000 debates, and I've seen clips of Bush's debates with Ann Richards. The general consensus regarding those contests is that Bush entered each heavily branded as an inarticulate, intellecutal underdog, and was able to turn the tables in dramatic fashion. He'd combine a joke and a smile with an easily digested, predetermined soundbite, and though never appearing brilliant, he'd always appear more in control than his stiffer, wordier opponent.
Yes, bias abounds, but the read I got from last evening's debate was of an attempt at the same strategy, but with fewer gains than expected. Perhaps that's due to Kerry's positioning as a double-reverse-underdog; so many expected Bush to run circles around him. The president's reliance on soundbites was more significant than usual; nearly every other question saw him erect a word bridge (sometimes smoothly, sometimes wobbly) to get back to an accusation of "inconsitent position," "mixed messages," or claims that the war was "hard." The repetition crossed an elegance threshold into sounding a bit dim. The Bush team seems to have found the limits of that technique; I suspect he'll repeat himself less often next week.
On top of that, Bush seemed far less comfortable than usual, and certainly less comfortable than he did against Gore or Governor Richards. He leaned forward, stuttered and paused frequently, seemed to lose his temper at points, and sighed loudly at least twice. These aren't cardinal offenses, but for a man whose key strength is his easy, everyman cool, they made a difference. Kerry kept his composure quite well, and the abrupt time limits prevented him from getting too... well, boring.
If debate victories are about strength of presentation, I don't think the poll numbers we're seeing regarding last night are a mystery.