It was about as nasty a debate as we have seen in this presidential cycle — and Mitt Romney was not even in the state.
I'm guessing that the tone of the campaign has changed because the Democratic candidates really do believe that their biggest competition is each other for the nomination, rather than the Republicans in the general election.
McQ at QandO has a thorough compilation of reactions to the debate. This one is particularly good, courtesy of the Carpetbagger Report:
[T]his probably isn’t the direction that actually benefits Obama. Clinton doesn’t mind getting into a good ol’ fashioned brawl; she’s quick, smart, and quite adept in these scuffles. Just as importantly, she’s not afraid to throw dirt — Clinton said last night of Obama, “The facts are that he has said in the last week that he really liked the ideas of the Republicans over the last 10 to 15 years.” That’s patently false, but it didn’t seem to matter.
The problem for Obama is getting dragged into the mud when he wants to aim higher.
I never thought that an Obama-Clinton ticket made sense--if he bests her in the primaries, then he doesn't need anything she can offer in the electoral college. And a Clinton-Obama ticket really doesn't make sense either--she already has problems being upstaged by her husband, so why would she want that from her running mate as well? But it seemed like bridges were burned last evening, and there's no going back (at least until the Republicans start replaying the most acrimonious exchanges during the general election campaign in the fall).