Some thoughts about tonight's debate:
Romney is not going to give up. Romney did not Pawlenty out in this debate: when struck at, he struck back---hard. The jobs exchange with Perry early in the debate showed some fight in Romney. Romney needs to fight Perry on this in order to defend his own claims on the economy.
Social Security and immigration are still issues. Romney's response to questions about immigration and amnesty subtly attacked Perry's support of in-state tuition for illegal immigrants and Perry's opposition to a border fence. After hitting Perry from the right, Romney also hammered Perry's claim that Social Security is a failure. Rather than abolishing it, Romney says he wants to save it. He's implicitly aligning himself with a long line of Republicans (such as Reagan and Cheney). Romney wants to challenge Perry's claim to be Super Mr. Republican.
Perry is not Obama. One of the dynamics of the 2008 Democratic race was that many Democratic candidates were willing to have Obama be the nominee if they couldn't. So they were quite willing to tear down Clinton in order to assist Obama. Going into this debate, there was a chance that other GOPers would view Perry as the preferred candidate. This debate showed that other candidates are quite willing to go after Perry. Many candidates jumped on the chance to pile on Perry regarding his executive order to mandate Gardasil vaccinations for Texas girls, for example. Perry does not seem to be getting the pass from Republicans that many non-Clinton Democrats gave to Obama during the 2008 race.
Newt plays the statesman. Newt Gingrich hit upon an interesting tactic in this debate: rather than feeding into intramural conflict among Republicans, he called out Obama and lashed out at the media. Both rhetorical tactics are sure to increase grassroots affection for Gingrich. By taking the battle to the president, he seems to rise above the fray while still seeming passionate. Gingrich had some strong moments tonight.
Ron hates Rick. While other candidates weren't afraid to knock on Perry, Ron Paul seemed to relish any chance to attack Rick Perry as a showman and pretender to conservative purity. Those attacks might not cripple Perry, but they could have an effect.
Bachmann struggled. Perry's entry to the race hurt Bachmann's momentum in Iowa and other states. Unlike earlier debates, Bachmann did not find a real moment to stick in viewers' minds. She's lost a lot of "Tea Party" ground to Rick Perry. If she wants to get that territory back, she'll need to shift the debate.
There were great expectations for Perry going into this debate, which seemed like it would focus on testing him. Though Perry handled himself fairly well, I don't think he landed a knockout blow. If anything, this debate perhaps strengthened Romney, as he was able to hit Perry from both the center (on Social Security) and the right (immigration). Tonight suggests that Perry is not invulnerable in the primary. Moreover, this debate also shows that other candidates are not willing to let this race become a two-man battle.
Update: Ace seems to agree with much of this analysis here.