Stepping out onto a very shaky limb here, I will tell you my opinion. Hopefully, I won't be flamed for having an opinion that doesn't match someone else's.
To answer the OP's question: I think that a lot of people who are Anti-Obama or even Anti-McCain fail to look at the president as one of a number of people in the Executive Branch. The president, whoever it is, does not act (unless it's Bush, LOL) all by himself, without advisors, without Congress, without consideration of consequences. The "checks and balances" that we learned about in high school civics are there for a reason--if, say, for the sake of discussion, Obama got a bee in his bonnet and wanted to raise taxes 500%, there are other people involved who would not tolerate that. Even if he vetoes something, if it's really far out, Congress can override the veto, assuming you voted for the right congressperson.
There are things about every person that are not particularly likeable. And when I look back on my own personal history, the decisions and associations that I made twenty years ago--I can say that I have regrets, I made mistakes, and I learned from them. I admire someone who can say the same thing. I am still making mistakes. I will be making them until the day I die. And so will Obama. And so will McCain. I did not vote for Obama because he is perfect. I did not vote for him because I think that every idea he has is right. I voted for him because in the grand scheme of the little that I know about the way the country works, I believe that things would be best if he were elected.
Don't get me wrong, though. I really like John McCain. He is a hero, and a true gentleman. I don't doubt that he would have made a good president, too. But coming on the heels of Bush, and seeing the way the world view of our country has changed for the worse, it was bad timing for McCain. I was just thinking to myself today that it has been a long time since there were actually two candidates that I liked. And I will never put down someone who RESPECTFULLY disagrees with the choice I made.