My reasons, and let me start off with stating that I am an independent:
1. The first time I heard Obama speak, I realized that he had a level of charisma that was above that of Hillary and the other potential presidential nominees. He sounds intelligent when he speaks; he sounds thoughtful, not quick to jump to conclusions. I want someone like that in the presidential seat. I feel he may be able to bridge the gap that has formed over the last 8 years between America and some of the other nations around the world and gain us back some respect. When I traveled to Europe around 2003, I was ashamed to admit I was American. People who had loved America just a few years prior suddenly were laughing at us for picking an idiot like George Dubya Bush. I hate the fact that the GOP is picking idiots to run for office this past few years to try and appeal to the common man. Intelligence, patience, and charisma to me are the most important attributes in a candidate, whether he be Republican or Democrat.
2. Ideas of being socially liberal, but fiscally conservative. The Bush administration went on a spending spree and put the credit card balance on the nation's back, in other words fiscally liberal, then clamped down on individual citizen rights, so socially conservative. Obama has laid out plans to trim the federal government back in certain areas so that we can reinvest into education and infrastructure.
3. I agree with his tax plan. Peel back the tax cuts Bush handed out to the rich, and give a tax break to the middle class, which gives more room for people to spend thus increasing the economy's health as a whole. I don't make a six figure or 7 figure salary like most of Anandtech. I have to carefully watch my finances and plan everything out just to get ahead. Obama's tax plan would benefit me directly in a tangible way.
4. Backing from strong military minds, such as Colin Powell, and General Petraeus. Bush had this habit of appointing people and then just letting them go wild. Obama promises a more structured approach. In other words Obama sounds more like someone who is fit to lead a business as well as the country. Petraeus stated that he may have not agreed with all of Obama's points, but that Obama sat down with him eye to eye and discussed why he feels the way he does, and what his plans are, and they worked it out. That indicates a strong military and tactical mind.
5. I agree with his enforcing of our internal infrastructure and scientific development. This is something that would benefit the country as a whole, and bring us back into the forefront of scientific development. Those in the science fields that I know of also support Obama for this reason. If we keep up our current direction we are going to start falling behind other countries in terms of science. There are a lot of extremely interesting technologies coming out, and now is the time to invest into our nation's future in my opinion. Example: carbon nanotubes, buckeyballs, etc. - we have nearly unlimited resources to make it, and they are finding more and more uses for it every year. We could be in the forefront of it's development, or we can sit back and watch while other countries move ahead of us.
6. Business - he wants to bring back policies which benefit the workers in some of the core industries of the US. They in turn support the local micro economies of the cities they are in, which brings in revenue to the state, etc..
7. Healthcare plan - simply better than McCain's. I'm not advocating universal healthcare, but let's face it, our healthcare system needs an overhaul big time.
8. McCain himself - notice how he's number 8, but the 7 things listed above are more important to me, but having looked at McCain's policies, and his choice of a running mate, I simply cannot approve of McCain at this point. He's pandering too much to the religious groups and fringe voters, and is starting to drown in a pool of his own bullshit. The McCain back in 2000 seemed like a decent pick; had he won the primaries back then I would have voted for him instead of Bush. Now that he has flip flopped so many times I lost count, I cannot trust what comes out of his mouth, especially given he hired the same people that slandered him and his family back in the year 2000. There's enough negatives on McCain right now to write another 100 pages.