I personally think Obama is a Marxist, but I understand why Mitt doesn't feel the need to call him names. Sarah Palin declined to go there as well.
Sometimes, rather than calling names and using ad hominem attacks, it is more helpful to give specifics and to discuss why Obama's policies are wrong-headed and explain how they will hurt our country. That's exactly what Mitt has done consistently in interview after interview.
Mitt is not out there right now trying to energize the base (that's Sarah's job #1). He's out there trying -- not to offend -- but to entice dems and indies in swing states to vote for McCain/Palin - especially when he's talking to a national audience on CBS Early Show(how many conservatives what that show - ever?)
For many people it is more effective to show how someone's actions will lead to a certain result rather than labeling someone and calling them names.
Also, I am sure both he and Sarah would be a tad less diplomatic in a small GOP group unlike when talking to a national audience with plenty of dems and indies listening.