I guess the main reason people dislike Twilight is that it took the pretty niche genre of 'Vampire flicks' and turned it into something that 12 year old girls could relate to.
If you're a fan of The Lost Boys (1987), or Vamp-lit authors like Anne Rice, then 'vampires that glitter and look like they could be in a boyband' come across as a bit too commercial and unauthentic.
To many people who were fans of vampire movies and culture, it seemed like their interests were being ripped off by Hollywood in order to make a lot of money.
I've not actually seen any of the series, so I can't really judge - but I can understand what it feels like when a subculture you feel a part of gets ripped off by the mainstream...