What would you say is the prinicipal reason for Daisy's appeal to Gatsby?

I would say that there are two major reasons for this.  I would say that part of the attraction is simply love.  Daisy is clearly an attractive woman, whether in Long Island or when she was younger in Louisville.

But perhaps the more important reason (so I'll say this is my choice for the principal reason) is that Daisy represents what Gatsby aspires to.  She is a rich, high class person.  Gatsby wants to be in that category.  So, when her pursues her, he is showing that he wants to be a rich, high-class person.  If she will accept him, then that shows that he has made it.