Reasons for Obama

1. Meta-politics
I think our top priority should be to strengthen democracy: improving the political system to make it more responsive to reason. That means increased transparency in government, ethics reforms to reduce the influence of lobbyists, and modelling intellectual honesty and civic virtue in political debate. On each of these grounds, Obama is the better candidate by far.

2. No More Torture
Here I defer to Katherine's expert judgment:

Neither [Clinton] nor Obama is good enough about accountability for past abuses; I think he probably is good enough going forward, but she isn't.

Update: see also Habeas Lawyers for Obama:

Some politicians are all talk and no action. But we know from first-hand experience that Senator Obama has demonstrated extraordinary leadership on this critical and controversial issue...

3. Iraq
Obviously. Here's what Obama said in 2002:

But I also know that Saddam poses no imminent and direct threat to the United States, or to his neighbors, that the Iraqi economy is in shambles, that the Iraqi military a fraction of its former strength, and that in concert with the international community he can be contained until, in the way of all petty dictators, he falls away into the dustbin of history. I know that even a successful war against Iraq will require a US occupation of undetermined length, at undetermined cost, with undetermined consequences. I know that an invasion of Iraq without a clear rationale and without strong international support will only fan the flames of the Middle East, and encourage the worst, rather than best, impulses of the Arab world, and strengthen the recruitment arm of Al Qaeda. I am not opposed to all wars. I'm opposed to dumb wars.

Here's why it matters:

[T]his isn't just about the past, it's about the future. I don't talk about my opposition to the war to say "I told you so." I wish the war had gone differently. But the reason I talk about it is because I truly believe that the judgment, and the conviction, and the accountability that each of us showed on the most important foreign policy decision of our lives is the best indicator you have of how each of us will make those decisions going forward.

How we made that decision, and how we talk about it, is critical to understanding what we would do as President. Will we carefully evaluate the evidence and the consequences of action, or will we skip over the intelligence and scare people with the consequences of inaction? Will we make these decisions based on polls, or based on our principles? Will we have the courage to make the tough choice, or will we just choose the course that makes us look tough?

4. Effective Diplomacy and Consensus-building
From international to local politics, Obama is willing to bring to the table those he disagrees with. And you know what: it works. Over to Mark Schmitt:

One way to deal with that kind of bad-faith opposition is to draw the person in, treat them as if they were operating in good faith, and draw them into a conversation about how they actually would solve the problem. If they have nothing, it shows. And that's not a tactic of bipartisan Washington idealists -- it's a hard-nosed tactic of community organizers, who are acutely aware of power and conflict. It's how you deal with people with intractable demands -- put ‘em on a committee. Then define the committee's mission your way.

This point also comes out vividly in Obama's in-depth Chronicle interview (highly recommended, especially if you want a better understanding of how Obama would go about things as president).

5. Competence
Many people complain that Obama doesn't have Hillary's "experience" in Washington. But anyone who doubts his wonkish credentials should simply take a look at his record:

[W]hile Obama has not proposed his Cosmic Plan for World Peace, he has proposed a lot of interesting legislation on important but undercovered topics. I can't remember another freshman Senator who so routinely pops up when I'm doing research on some non-sexy but important topic, and pops up because he has proposed something genuinely good. Since I think that American politics doesn't do nearly enough to reward people who take a patient, craftsmanlike attitude towards legislation, caring as much about fixing the parts that no one will notice until they go wrong as about the flashy parts, I wanted to say this.

Follow the link for the details. (Then follow all the previous links in this post!)

Overall: Compared to Hillary, Obama is more electable, more likely to be able to effect change once elected, and the changes he proposes are the right ones. Why in the world wouldn't you support him?