I have a feeling that the candidates focus groups iterated a distaste for heated exchanges between the candidates and they scaled back any notion of direct vociferous debate and tried to appear "positive" instead.

If you're in the forefront like Romney, who's been running for President for five years, this is a godsend. The other candidates needed to highlight their differences with Romney in a way that intrigued the viewer, and all of his opponents failed miserably in this last debate to provide that viewer epiphany.