For starters, it follows the basic rules that apply to vampires, unlike Twilight where Edward Cullen just goes into Bella's house without any invitation, etc.
What's also gripping is the fact that it has other characters from the magical world, like Witches and all. It also sticks to its plot, so what we are seeing is actually a story which has vampires as a part of it, not the other way around.
Definitely worth a try, and one of the best shows of Fall '09.
And the coupling of it with Supernatural on Thursday, making it a Thrilling Thursday on CW is quite cool too! A 9/10 for me!