Here's my reasoning for voting for McCain, as of now. It's still early yet, and these may change.
1) The Senate will almost certainly have a Democrat majority, as will the House. I do not like and never have liked the idea of one party in control of both. I think that a lot of Bush's missteps, especially in regard to Iraq, had a lot to do with poor Congressional oversight. Yes, some Republicans disagreed with him on some issues, but there is a reason that he didn't veto anything until 6 years into his term, when there was suddenly a Democratic Congress. A President unchecked is a bad thing, whether it's Republican or Democrat.
2) I like the idea of having someone who actually served in the military running the country while we're in Iraq. I don't care about the rank, just someone who has had the experience. Combat experience is a plus. Perhaps it's just me, but I trust someone who has been to war when they talk about war more than someone who took deferments and flew planes once in awhile in the National Guard. McCain might keep us in Iraq, but I'm hoping that the situation will be managed a lot better than it has been in the past (though I think that a lot of that lies more with Rumsfeld than with Bush).
3) McCain is not popular with the NeoCons. That's a plus in my book. The old fashioned conservatives I can live with, as we have some beliefs in common, but the NeoCons are another matter. Perhaps Colin Powell would come back to the government.
4) He's old. He has nothing to lose, and can do as he wants, within reason, as he'll theoretically be constrained by a Dem Congress. He won't have to pander to the fringe of the Republican party. I tend to think that he'll be more bipartisan and may have more of a willingness to work with the Congress than Bush has, perhaps this is because Bush was a governor and McCain a Senator, I don't know.
I will be far more interested in either candidate when the primary season is over and they both begin to pander to the center. Right now they're just preaching to the choirs, it'll be interesting when they have to preach to the populace.
I've said before, and I'll reiterate, if Richardson becomes a VP nominee I will probably vote for that ticket, even if Hillary is the headliner. I do think that if Obama wins the nomination, and picks Richardson as a VP, that it'll all be over.