As Elephant Watcher pointed out in Romney's Profile, Romney's main challenge will be dealing with Romneycare. Certainly Romney has some other weaknesses, such as his history of flip-flopping, being distrusted, and his Mormon faith. None of them compare to the problem of his instituting a policy so similar to the reviled Obama health care plan. After all, opposition to Obamacare was what united the Republican opposition leading up to the 2010 midterms and fueled the Tea Party. But at the debate, it appeared to be little more than a speed bump.
Nor was Romney criticized by the other candidates in any way. He is currently leading in all the state polls, but the rest of the field didn't lift a finger to challenge him. This leads one to ask the question: Are they really going to let Romney get away with this?
In a word, no. The first debates do not normally reflect that dynamics of a long presidential primary. Candidates are much less willing to attack each other. In the 2008 primary, for example, Barack Obama repeatedly refused to attack Hillary Clinton for several months--even though she was the undisputed frontrunner and 20+ points ahead of him in the polls. But as the date of the Iowa Caucus drew near, the criticism of Hillary intensified. In the end, Hillary lost Iowa. The Republican debates were not so different. They were very civil, and few candidates wished to speak ill of the others. At first. But it didn't last long. By the time South Carolina rolled around, Fred Thompson was practically accusing Mike Huckabee of being a Democrat.
There are a number of reasons why the candidates play nice in the beginning. No one wants to be known as the candidate who "went negative" first. Nor does anyone want to shoulder the risks of attacking the frontrunner while everyone else shares the benefit of the frontrunner being taken down. It's also important not to alienate the supporters of the frontrunner, who you'll eventually need in your coalition.
Long story short, the campaign is going to get rough several months down the road. Candidates will be more than willing to unleash their ammunition against Romney--especially if he still leads in the polls. Moderators of future debates will likely ask Romney about Romneycare again and again (unless Romney gets lucky). The issue will also be brought up because the constitutionality of Obamacare is being determined by appellate courts (and later, the Supreme Court). That will keep Obamacare--and therefore, Romneycare--in the headlines during the primary.