It’s extremely hard for any candidate to be so awful that he gets much less than 40% of the electorate (in a two party vote, of course). That tells me that about 80% of the population have their votes set in stone routinely, no matter the name on the ballot. (Of course, individuals change their affiliations, roughly counter-balancing each other, from Republican to Independent to Democrat). The other 20% determines the election. So roughly 40% of the electorate is voting for McCain because he’s got that ® next his name.
The interesting part of your question is: why would anyone over and above McCain’s base vote for him? Why would an intelligent, careful, thoughtful Independent voter decide that McCain looks like a good choice? Maybe because they’re not all really intelligent, careful, or thoughtful? Maybe they’re not all that Independent, either? Maybe they’re one-issue voters, who might vote for a Democrat if he came out strongly against gun control while the Pubbie took a weaker stance, but in this case have decided that McCain’s the Guns Yes! candidate? Maybe they don’t know shit? Maybe they’ll pull the wrong lever, or push the wrong chad? It may be a million or so voters who fall into any of these categories, but that’s a pretty small slice of the voting public.