My assessment of the debate:
Obama was sharp on the first question and then became a little flat. McCain came out firing and was aggressive and went after Obama. Obama refrained from taking the bait, but he should've snapped back a little to show some fiery passion. The tone changed when Bob Scheiffer asked the question about negativity in the campaign. Obama took a more mature high road and essentially said that it's part of politics but we need to move past it because it doesn't help the American people. From there on, he came off as cool, calm, collected and therefore, more presidential. McCain, however, lost his message. He drilled the attacks and the negativity and still tried to paint Obama as inexperienced and risky, which to this point, hadn't been effective. From that point on, Obama really got back to the issues he's strongest on: the economy, health care, education (scored big points here). He also had a vision for the future. On an unrelated note, when the screen was split and Obama was talking, McCain looked like the grumpy, angry, old guy that everyone accuses him of being. He looked stiff and had pure contempt in his facial expression. Image is very important on camera. Gore in 2000 got a lot of heat because he just looked stoic and lifeless.
At the end of the night, Obama won it, but debates are never game changers. Bush lost all three debates in '04 yet he still won the election.