I think Yglesias has this pretty much right:
To me, the crux of the matter is that McCain can't get out of the habits that served him very well when he was a Senator building a glowing national reputation largely by talking directly to elite members of the political press. If you watched the previous two presidential debates, plus the VP debate, plus about half of the Democratic primary debates, plus the prime time speeches at the Democratic National Convention, and you've seen a dozen Obama surrogates yakking on cable a dozen times each just since Lehman Brothers went under then it gets kind of boring to watch Obama stay calm and repeat his talking points on the key issues.
But the debate is targeted at folks who haven't watched all that stuff. And a lot of McCain's best moments will have gone way over the heads of most people.
For example, he alluded at one point to a desire to allow more imports of sugar ethanol. Now if you're familiar with the details of the ethanol debate, you'll know that McCain's stance on this is correct on the merits. And you'll also know that Obama is a big support of corn ethanol both because they grow corn in downstate Illinois and because they made a big push for the Iowa Caucuses. McCain, by contrast, has a long and principled record on corn ethanol that's hurt him in Iowa. This isn't the biggest deal in the world, but it is a nice illustration of some of McCain's key campaign themes. And yet he didn't try to explain it at all. Similarly, he's had a knack for besting Obama on national security issues nobody cares about, like the relationship of US-Colombia trade deals to the US-Venezuela proxy conflict playing out in the Colombian jungle. People figure that Obama seems like a smart guy, and if something important happens involving a guerilla group nobody's heard of fighting a president nobody's heard of in a country nobody cares about, that Obama's up to the task of coming up with a good idea -- meanwhile, McCain has no education policy.
And JPod makes a similar point as well.