The main problem with Twilight is that you can completely take the fact that they're "vampires" out of the story and it wouldn't change a thing. You would get the same exact story. The fact that some of the character's are "vampires" Does not drive the plot, at all. At least, that's how I felt after seeing the movie.
It's true that there are no set "rules" to vampires. They're mythical creatures. But you know what else is myth? Greek and Roman gods. But if I were to make a movie about how Hades was really the god of flowers and cuddly teddy bears, I'd get reamed for it. There are no rules, they're myth, but there is a specific lore that goes with the creatures. Stephanie Meyer took all the aspects of vampires, urinated on them, and threw them out the window.
Now, I'm not saying you can't change vampires. One of my favorite book series when I was younger was Cirque Du Freak (Yeah, they just made a movie about it. Yeah, it sucked). In this series, the author described vampires differently than most would. In short, Vampires were not evil, they drank only a little blood from victims who they would knock out, ate normal food as well, were effected by sunlight if exposed to it for prolonged periods of time, and were not immortal (although they only aged 1 year for every 10). There are many other subtle changes but I won't go into them (There are also an evil clan of vampires called the Vampanese, these do kill when they feed). The reason I was able to enjoy these changes was because the Author was able to explain them in a very realistic manner and fit them into the plot well. Although they didn't fit every detail of vampires, they still felt like vampires.
Same can't be said for Twilight, once you've got "vampires" walking around in the day time (Hmmm, aren't they called the creatures of the night?) with only a shimmer to show for it, you kind of lose that feeling. To put it simply, nothing about the characters in this story are vampiric at all. Stephanie Meyer simply used the increasing popularity of vampires to sell another trashy teen love series about a whiny girl who just can't live with out her dear Edward.
But that's just how I feel.