Well, I don't know that this debate was quite as bad for Obama as the general consensus. He did get some core points across: that Romney's tax math doesn't add up, and that he's hiding the ball with regard to deductions; that a balanced approach, i.e. with new taxes, is a better approach to deficit reduction than cuts only; that Romneycare and Obamacare are structurally almost identical, and that Romney has no plan to cover the uninsured; that Romney proposes to voucherize Medicare.
But he didn't make the rebuttals concrete enough. He was too rigid in insisting that Romney wanted to cut taxes, instead of saying that his plan would either reduce revenue or raise taxes on the middle class or explode the deficit. In complaining that Romney had no real plan to cover the uninsured, he talked around the individual mandate, his real point being that when Romney was serious about expanding coverage, he pioneered the mandate. He didn't defend Medicare's Independent Payment Advisory Board effectively, or do any better job than the Republican presidential candidates did in exploding Romney's faux contrast between the healthcare plan he implemented in Massachusetts and the ACA. When Romney hit him for "ramming through" a plan with no Republican support, he didn't assert that the GOP sabotaged him, professed their desire to defeat him at every turn, demonized ideas that they'd pioneered (though he did say that the plan's main components were initially Republican proposals). He didn't defend his green energy investments at all, or rebut the Solyndra bullshit.
And of course, he didn't attack enough: didn't articulate how Romney's plans benefit the wealthy primarily, didn't drive home how revenue-free deficit reduction would gut domestic spending, didn't effectively rebut Romney's claim that he wouldn't cut Medicaid or other domestic spending drastically (he has to, if he proposes to balance the budget with no new revenues or with new tax cuts); he didn't mention the 47% comments or challenge the notion that the country is divided into makers and takers. And he smiled too much, and looked down too much, and yielded to Lehrer much more than Romney did.
The media narrative that Romney dominated is almost unanimous, and John Sides et al tell us that that drives public opinion more than the debate itself (though the opposite seemed true in many Bill Clinton speeches; is reception of speeches different in that regard?). So we'll see how much this moves the polls.
p.s. Fallows, of course, tried to warn Obama & co. of the debate dangers for incumbents; his quasi-forecast is infused in the post-debate commentary. And I'm disappointed that Obama did not follow his own script and call out Romney's lies and the brutal cuts silently inscribed in his sketched-in policies. Apparently Obama hasn't been tracking Romney's repositionings as closely as, say, I have. He's a busy man