First: it was a very good speech. From an objective standpoint, it was well-written and well-delivered, and she had the crowd in the palm of her hand. Second: she said she supported Obama, but she never got to why beyond the fact that he was the Democratic nominee. She never talked about his character or his substance. It was a hollow endorsement, and the speech was designed more to boost her than boost him.
Third: I would guess that many Democrats in that arena tonight, when the speech had ended and everyone was filing out, got a chill and suddenly thought, "Oh lord, we nominated the wrong candidate."