Daisy (Fay) Buchanan was a well-meaning, but ultimately weak and indecisive woman from a wealthy family. Modern critics might characterize as a Stepford Wife. Her main problem is that she doesn't seem to "take responsibility" for her actions (even though she appears not to mean any harm). In Nick's opinion,
"They were careless people, Tom and Daisy – they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made…" [Emphasis added.}
For instance, she falls in love with Gatsby, then is swayed by Tom Buchanan, then (reasonably) thinks better of her upcoming marriage to Buchanan, but without cancelling the wedding. Then Gatsby comes back into her life, and she thinks of leaving Tom for him, but finally opts for the status quo, just because it is the status quo. Finally, she gets into a hit and run accident with her husband's mistress, and doesn't stop to help or leave identification, which is basically illegal.
A hundred years ago, such indecisiveness was tolerated in, if not expected of a young woman. Not so today.