Other then the fact that he's solidly conservative, good looking, well known, fairly well spoken and a medical doctor?
I honestly can't ever figure out people who say Frist is a non-entity. He's no Ronald Reagan, but he's more charismatic and has a more attractive bio then quite a few other people who have been president. Certainly more so then LBJ, Nixon, Ford, Carter, or Bush Sr.
It's probably because no one is really fired up or passionate about his presumed candidacy. Frist strikes me as the type of candidate who is everyone's second or third choice - the safe alternate in case the top choice doesn't make it. There are all kinds of people on FR who are fired up about Condi '08 or Sanford '08 or even Allen '08. But I've yet to come across many people who are jumping up and down for Frist. That lack of buzz or excitement may cause people to downplay his chances.
Are you looking for a President, or a son-in-law?
Frist simply has no legislative achievements to his name. There is no Frist bill. He's a reliable vote, but has not even distinguished himself as majority leader. As Senator from Tennessee, there's nothing wrong with him, but there's no there there. He's well-spoken and pretty smart for a M.ediocre D.octor, but his resume hasn't much more substance than it did when he was first elected Senator, just more titles.