It's undoubtedly true that John McCain has some real strengths as a candidate, most notably the DC press corps' undying love for his wrinkly visage. That said, I'm not sure how much stock I would put into McCain's low unfavorables numbers. That's an advantage, to be sure. On the other hand, it's not a mystery where that advantage came from -- it's come from the fact that McCain's never been subjected to serious attacks from the left on the national stage. But it's not as if this pattern would continue were he to win the nomination.
Nor do I think it would be impossible to construct attacks against him. Matt Welch's book, McCain: The Myth of a Maverick, for example, contains plenty of the negative information about McCain and his record that's simply never been mentioned by his friends in the press. He'll continue to have that alliance, but in the context of an actual race it won't be possible to keep it all off the table. Meanwhile, McCain really lacks a lot of the characteristics you normally look for in a presidential contender. Like many losing candidates, he combines a lack of executive experience and the signature accomplishments that come with it with a long record of service in the Senate and the flip-flops and old votes that come with that. What's more, though he's obviously charming if you sit on a bus with him, the persona that regular people see isn't a charismatic one -- he's super-old and much more in a crazy old-man neighbor-guy kind of way than in an avuncular and reassuring way.
All things considered, if I were running a presidential campaign I'd rather not go up against a guy who'll be able to count on 75 percent of political reporters acting as de facto campaign volunteers, but it's not like he's some unstoppable force either. Unlike George W. Bush or, indeed, every successful national politician I can think of, McCain doesn't even pretend to care about the economic problems of struggling people.