Obama's greatest enemy tonight is his own ego. By making the speech on the forty-fifth anniversary of Martin Luther King's "I Have A Dream" speech, he is inviting the comparison. There's no way he'll let the evening pass without mentioning that he is the culmination of Dr. King's speech. This might strike some viewers as presumptuous; at any rate, it won't help with the growing perception that Obama's candidacy is less about his positions and more about him. Expect soaring rhetoric and vague policy. After scanning some excerpts that the campaign released ahead of time, I noticed that Obama now says he supports cutting capital gains taxes for some businesses. That's a bit of a change from earlier this spring, when Charlie Gibson school him on the subject during the final debate of the Democratic primaries. Don't be shocked if more contradictions like this pop up - after all that's what happens when you try to be all things to all people.
Also expect him to try to maintain a positive tone. Remember, in his mind this is all about him, and I think he's more likely to talk himself up than talk McCain down. That's not because he's got any aspirations to maintain a positive tone in the campaign; it's just because he won't even be thinking of McCain, as he'll be too busy basking in the adoration of his screaming fans.
Lastly, expect a speech that makes people feel good for about two days and then fades from all memory. It'll be Berlin or the Philadelphia Race speech all over again; people will fall all over themselves praising it, but when it's all said and done, there won't be one line that people remember unless they're mocking it.