Yuval Levin, I think, hits the nail on the head:
What it didn’t do, though, was answer the basic question the McCain campaign has worked to plant in voters’ minds: what makes this guy think he’s qualified and ready to be president? If the question has really sunk in, Obama is in no less trouble than he was before this convention, and the Republican convention—where McCain’s readiness and experience will be front and center—will do him real harm. But if it hasn’t, and if what people basically want is to get away from the Bush administration, he has certainly given them a reason to see him as their guy.
But there is something else: Barack Obama struggled mightily during three days plus of the Convention to convince voters he is sweet, considerate, and just like them. The speech isn’t going to make voters like him or identify with him. But maybe that’s all hooey and the point is just to get voters to hate George W. Bush and the Repbulicans. If so, he may have succeeded.