Daisy is not as great as Gatsby thinks she is. First, he has inflated her in his mind to such mythic proportions that no real woman could possibly live up to his expectations. Nick notes this on the afternoon that Daisy and Gatsby are finally reunited after five years:
Almost five years! There must have been moments even that afternoon when Daisy tumbled short of his dreams--not through her own fault, but because of the colossal vitality of his illusion. It had gone beyond her, beyond everything.
Second, even if Gatsby had not built Daisy up in his mind as the symbol of all his desires, she simply was not a person of great character. She was weak. Near the end, after her flirtation with Gatsby, she went back to Tom, despite all his adulteries. As Nick puts it:
the dead dream ... slipped away ... whater intentions, whatever courage she [Daisy] had had, were definitely gone.
At the end of the novel, Daisy lets Gatsby take the blame for running over Mrytle, even though she was the one driving his car.
Finally, she doesn't show up for Gatsby's funeral, even though he arguably laid down his life for her. Nick writes:
They were careless people, Tom and Daisy--they smashed up things and creatures and then retreated back into their money ...
The novel might show Daisy to be unworthy, but it both exalts and critiques the idea of having a dream: in fact, such dreams, for better or for worse, are at the heart of what America is and has always been, Nick Carraway maintains.