My observations from the debate and from the campaign thus far:
Mitt Romney: He's honestly one of the most plastic candidates I have ever seen (and I lived in the state from whence Al Gore came!). Seriously, he's like watching a really bad actor trying to get by on looks and cue cards. I'm not trying to insult any of his supporters, but just as an observation I don't think he comes across as a guy who relates to people unless you're someone who is a member of a really expensive Country Club and you sit around at the bar discussing last weekend's jaunt to the Riviera.
Rudy Giuliani: To me, he doesn't come across as a professional. Don't get me wrong, you don't want to make the Romney error above, but if we had a Westminster system, Giuliani would make a great "attack dog" for a party. Anyone familiar with Westminster politics knows that the attack dogs never become Prime Minister (though occasionally they get a cabinet post as a reward). I also believe he's made a very fatal mistake in adopting the "ignore Iowa and New Hampshire" strategy. Gore tried it in '88, and others have tried it since. It doesn't work and I don't expect Rudy to break the mold.
John McCain: Likely dead in the water if he hadn't won New Hampshire, McCain is leading in Michigan polls, a state where Romney should have a large advantage. McCain is likable. He is a maverick and Americans always like a maverick, at least for a while. He's energetic for an old man, and has a great sense of humor along with a dazzling war record. His problem has historically been that in closed primaries the true Republicans don't love him near as much as independents. He may solve that through "electability" as he is the only GOP candidate who polls ahead of Clinton and with Obama in national polls.
Fred Thompson: A cliche. Thompson is actually a very intelligent lawyer, but he's presenting himself as the good ol' charming country boy from Tennessee. He has nothing that sets him apart from the other candidates other than a few remarks on state's rights, which is not a major vote winner across the nation. Also, W has played the charming, simple, Southerner card in two elections in a row and I don't think Republicans will pick up that card another time. As for skipping Iowa and N.H. - see Giuliani above.
Mike Huckabee: Likable, and like McCain he has a great sense of humor. He's a Baptist preacher, but does a good job showing the softer side of his faith without a lot of the rough edges. His weakness is foreign policy and the question is whether that will hurt him or help him. I think that is the sharp contrast between Huckabee and McCain.
My thoughts as a whole: I really see this as coming down to Huckabee, McCain, and Romney. Huckabee and McCain will be a real choice on the Iraq War, War on Terror, and how best to wage those battles. Romney is the safe plastic person to choose if either of the other two should falter.