Originally, I thought this was going to be a difficult choice. Luckily, events have unfolded in such a way that I can simply rule out those candidates that are completely unsuitable and unacceptable as reasonable candidates, and then vote for whoever remains (even if they aren't a great option).
Now, let me precede my thoughts with the note that I am a registered Libertarian, so I hate both the Republicans and the Democrats, generally equally. I would lean towards voting Libertarian (as I have in the past), but unfortunately, I think Bob Barr is an idiot. He was a complete moron when he was a Republican, and even though he's improved a little bit, he's still so far from reasonable that he's not worth wasting any more thought on. (Although it still annoys me that he was selected as the Libertarian candidate.)
A few months ago, I was very undecided on who I liked better, Obama or McCain. Then a few things happened that pushed me to one side.
The first was McCain's appointment of Sarah Palin. Sarah Palin is an idiot. Every time she opens her mouth, she further confirms that she should never speak publicly again. Worse, John McCain is 72 years old, and the average life expectancy in the US for men is approximately 75 years. That means that statistically, McCain will most likely die before he finishes his term as president, if elected. And I think Sarah Palin is so woefully underqualified for the Vice President position, let alone the president, that this possibility seriously scares me. As far as I'm concerned, she was a horrible selection, and the simple fact that she was chosen puts McCain's judgement in serious question to me.
The second was the negative campaigning. There were a few pot shots here and there from both sides, but the first time I saw it really escalated was when Sarah Palin was announced. She came out and was immediately on the attack. . . and I'm so completely sick of that. Since then, as McCain started slipping in the polls, his campaign actually *announced* that their new campaign strategy was to be more focused on attacking Obama and they ramped up the negative campaigning to a new high for this election. They announced that. In a press release. What the hell?!?!
And this was after McCain pledged to run a clean campaign. Heck, early on he *did* do a great job with that. I had a lot of respect for him during the primaries, and early on when someone outside of his campaign initially started some negative campaigning, and McCain jumped in and shot them down, saying that there was no place for that kind of negativity and attacking in his campaign. That was integrity. That was impressive. Unfortunately, that was then.
Another issue that pushed me away from McCain was his proposal of $300 Billion in direct-to-house-buyers bad mortgage buyouts/bailouts. Basically, he wants to spend $300 Billion to buy up mortgages for people who are financial idiots and got in over their head, didn't plan properly, didn't save properly, etc. Then he wants to give them a new mortgage based on their house's current value. Because it's my responsibility (as a taxpayer) to help someone else pay for their house?!? What the hell?!?
I'm sorry, but if you bought a variable rate mortgage when the interest rates were low, and didn't have a plan in mind for what you were going to do when they (inevitably) went up, and you're now screwed, that's your problem, not mine. If you make $35k a year, and your bank approved you for a $250k house loan, and you bought a $250k house (without taking two minutes and the back of a napkin to figure out that you can't afford your monthly mortgage payment on $35k/year), that's your problem, not mine. If you bought your house for $250k a few years ago, and it's now only worth $150k, I sure as hell don't want to pay for the difference, and I'm amazed and horrified that John McCain would suggest that we should. That's your problem, not mine.
Now, for anyone thinking this is a McCain bashing, that's not my intent. I may not be able to Sarah Palin, but I don't hate McCain. In fact, there's a lot of things I dislike about Obama, too, and if there was a decent Libertarian candidate, they'd almost certainly get my vote. But John McCain has thrown away any and all chance he had of my support. Due to his extremely poor choice of Sarah Palin, his flip-flop choice to take the (relatively) civil campaigning to a new low for negativity, etc, he's no longer a viable candidate to me. Bob Barr was never a serious option. That means that even though I have a few concerns about Barack Obama, he'll get my vote because he's only remaining candidate that I'd be willing to consider voting for.
I also find it rather interesting that Colin Powell has endorsed (and strongly so) Barack Obama. He makes a pretty compelling case, too. I mention his endorsement just because I think he's a man of some integrity, even though I've frequently disagreed with his policy opinions. If you haven't seen/read it, it's worth a look. Another well written statement comes from Tim O'Reilly, of O'Reilly books (well known among most techies and geeks as some of the best technical books available).
 No, I'm not an actuary. Yes, I'm aware that my statement about McCain's likelihood of dying in office is a gross oversimplification that doesn't take into account the many positive aspects (rich, relatively active, excellent medical care) and the many negative aspects (four bouts with cancer, including high risk melanoma, advanced age, rough times in his life) of his health. I don't care. I have to go on the information I have available, to the best of my ability.
 Yes, I realize that there's a small percentage of people who are getting hit with bad luck, bad economy, downsizing, etc. And that's unfortunate. But there's way too many people who dug their own hole. Hell, I even have a friend or two that are in a really bad spot regarding their mortgage. And I feel really bad for them, and I truly hope they come through it okay. But in both cases, they consciously made the decisions that have put them where they are, and it was entirely their own choices. One of them bought a house that was, from the beginning, more than he could afford. I've never understood what he was thinking, and never really thought he'd find a way to make it work. The other gambled on housing prices continuing to increase, and lost. Never gamble with more than you're prepared to lose.
[Note: Observations and comments are welcome, but please don't try to turn this into a political debate. At this point, I've made up my mind, and I don't plan on changing it due to an online argument. ;-) ]