I posted a long while back why I don't see Romney as a smart choice as the GOP nominee. One of most apparent reasons is his history of flip-flopping at politically convenient times.
In this article in US News, Liz Halloran explains why it's still a problem for him, and why it's not getting any better. The debate Wednesday highlighted the fact that Romney still can't find a straight answer for the questions on his past positions. Whereas Giuliani seems to have put together a plausible story as to why he's wavered on certain issues (citing states' rights and such), Romney still equivocates.
Because Romney has positioned himself as the socially conservative candidate...the option for the Religious Right...he's having to try harder to keep from alienating that base. I assume this is the cause for all the circumlocution. Now when it comes to the general election, swing voters might not care as much what his position is, but they want a candidate who can stand up, look you in the eye, and tell you straight out what he believes. So far, Romney hasn't been able to do that.