What is the main reason for Daisy's appeal to Gatsby?

Aside from Daisy being attractive I would say that Gatsby is in love with the idea of Daisy rather than Daisy herself. Daisy represents a fantasy girl that is not real. Gatsby is rather immature that way because what he wants does not exist. Daisy is part of the crowd that Gatsby aspired to. The rest of how Gatsby sees Daisy is an illusion.