I dislike her in part because of her attitude. She kinda throws herself at Cloud for reminding her of Zack, and just fits a lot of gender stereotypes. She's also physically weak, making her not my favorite character to use in battle.

Lastly, on my first playthrough I just thought Tifa was the best person for Cloud. That seems to be what most of this debate comes down to: whether you like Tifa or Aerith for Cloud.