I mentioned that I'm not that big on vampire stories. Neither is my co-worker. She nearly said all vampire stories are stupid until I pointed out whether the stories are good or not depends on the mope writing the story. For example, Love At First Bite is a vampire movie, but it's done in a unique way. Shadow Of The Vampire is another. She started leaning towards my point.
In many ways, the appeal of Twilight isn't such a mystery. Or Gossip Girl. Or most anything else out there. And it comes down to a war within the story structure between audience identification and audience fascination.
As a writer, your whole job is to engage the audience. You can do this with a dynamic plot, sure. That's how disaster movies work -- the characters don't really matter, just who gets whacked, who survives, and how this happens. When I first started writing, I tried to keep my characters as generic as possible and have the plot do the heavy lifting, thinking it would make it easier for readers to live the adventure through the character. After all, there was no "But I wouldn't do that" to break the illusion of the story. But it didn't work, a hard lesson to learn. (The result of my capitulation to this fact was a novel I wrote called "Maelstrom", a superhero story that marked my first serious attempt to create personalities for my characters and have them interact in a realistic fashion. I thought I did pretty good at the time, although, since I haven't read it in over a decade, I can't tell you whether or not it holds up.)
So, you need to make interesting characters. You do this by either making them so that the audience can identify with them or by making them fascinating. Either way, readers have to want to follow along. In Stress Puppy, Raff is identifiable, either by knowing someone like him or identifying with him. He is the voice of reason in the strip, the one trying to maintain an even keel with the unfolding events. There might be some mixture of wish fulfillment and cynicism in there, as his voice of reason is typically ignored, and he has to resort to subterfuge, trickery, and manipulation to fix situations that have gotten out of control. If things were idealized, he could simply explain his wisdom and everyone would see how right he was and so on. It works on family sitcoms and other shows like that, but think about it -- those shows exist by presenting an easy rhythm viewers can get into. Nothing too harsh to prompt them to tune out because it is too much of a reminder of the lives they are trying to escape from for a little while. TV shows have to be bland and populated with Mary Sues. Making the audience pay attention is not how the contract with television entertainment works.
The other way is to construct a character who is fascinating. I think you can sort of mix the two, but not too much or too well. After all, if the character is fascinating, you want to see what they will do next. In short, it's not something you would think to do, which means you don't identify with them, or else you know what you (and by extention, the character) would do.
See what I mean about the war?
Now, this also applies to villains or general antagonists. The bad guy whose life sucked just like yours and decided to chuck it all (the central premise of Wanted). So many people live for themselves, maybe it's time they took what was theirs instead of waiting for it to possibly be given to them. This is what powers the Gossip Girl series, as girls know somebody just like all the main characters (which I also find a depressing commentary on what social life for girls is like, but I digress). In the realm of fantasy, though, you have shorthands that you can use. And what makes them fascinating is their metaphorical relation to the viewer.
Let's start with vampires, as that is what started all this. Twilight, simply put, is American shojo. Nothing wrong with this, and in fact, vampires pop up with some regularity in shojo, most recently with Vampire Knight (just went into production as an anime after the manga drew huge numbers). Whether you agree with Stephen King or not that the writer of Twilight has no talent for writing (he blasted her and suggested she learn a few things from J.K. Rowling, who I agree is one of the best contemporary writers around), it is striking a vibe. First, the Mary Sue central character (see my note on genericizing your viewpoint character above). But then you have vampires.
What's the appeal of vampires? It's the metaphor, baby. Vampires live a night life, they not only don't but can't live normal "get up and go to work" lives, there is often some sort of status (remember, he was COUNT Dracula, and vampire books frequently have vampire societies with their own rules and punishments that are more immediate, logical, and satisfying than normal societal laws), they are stronger, have a more worldly experience, and (and this is the most important part) they are sexually irresistable. So much so that women will cast aside their primary protection (a vampire cannot enter a house without permission) just because they are so enraptured. Frequently, who we choose to chase is founded on the status such a catch will lend to us. "Look at who has opted to spend their time with me. Die jealous." Guys will brag to their friends about beautiful women they've bedded. Comic book fans at the ChicagoCon, if their girlfriend has any sort of build, will parade her around, with her wearing some sort of outfit showing off her shape (and if he can get her in, say, a Supergirl costume, bonus). Meanwhile, they will look like Silent Bob. The year Jay And Silent Bob Strike Back and the year after, I saw this dynamic more times than I could count (some guys even dressed just like Silent Bob). Women also look to status -- someone they can brag that they landed. It's not love they are after, but the jealousy of people who see them together.
A lot of your classic monster and horror movie tropes are borne of the audience's identification. It was years before I realized there were actual werewolf legends. I had figured the wolfman was simply created as a metaphor for guys going through puberty (nighttime comes, hair grows everywhere, they become animalistic, and either abduct women or fight with other men. That's either a guy going through puberty or your average pro football player). Mentalist villains wrest control of yourself away from you, forcing you to do things you wouldn't do like commit crimes. It's no surprise that one of the largest subgenres of erotic fiction is the mind control scenario, as it also provides its own buffer -- you're not really a slut, because you aren't the one doing this. Even if you enjoy it, you would still be upstanding and resist, but this guy is making you (yes, there are female mind controllers, but they typically aren't as malicious as the males. I've read only a handful of erotic stories with female mind controllers that turn their victims to depravity. Usually, it's more to win them over and let them live an idyllic life, not controlling what is not given to them). It has its own absolution for fantasies the reader might not be completely comfortable with. By way of contrast, part of what makes zombies so terrifying is the thought that you lose your ultimate property -- your own body. You get kicked out, no way back, and what's left of you literally belongs to someone else. If you look at the trends of what movies get made and are popular, zombie movies usually come into vogue during periods of social domination (the Moral Majority in the 80's, the Patriot Act of the turn of the century) and fall away when such social strictures are more relaxed (they didn't appear as often during the Clinton administration, for example).
You have to make sure, however, that you communicate the horror of the situation clearly. One of the things I was hoping I did with The Supremacy was the character of Pulsar. Pulsar I tried to make like every person stuck in a job they just can't leave. We often forget how queitly confining life is. People will work jobs they hate simply because it pulls in a good paycheck and provides stability. Changing jobs often is just a lateral move, switching ladders instead of climbing up one, and it means a whole new set of rules for interaction to get used to, with the possibility that won't be as managable as the little niche you've carved out for yourself here. Pulsar has an additional problem. Creutzfeldt has not only stolen Pulsar's identity and life, but is also after his sense of self. Pulsar's only other option is homelessness, where anything that makes him who he is (what he likes, what he dislikes) MUST be disregarded if he wants to survive. It's about going from one day to the next. No connections, no friends, no promise of tomorrow, nothing, just another piece of driftwood in the ocean of life. Sort of like the end of "Down And Out In Beverly Hills," where the bum went back to the family because he would rather be stuck there than hunting and gathering again. Freedom wasn't as important as belonging to a world where he could exercise his will. For someone like me, being forced into giving up your options to do as you wish is almost a living death.
And the trade-off continues. At what point do you give the audience someone to root for because your character is them (Columbo is the detective as Everyman, Jim Rockford is the detective as Anyman), because they are realized on their own and the audience bonds with them as friends rather than proxies (Adrian Monk, Harry Dresden), and when do you give them something that speaks to their inner desires and fears instead of a character? It's something every writer has to figure out, because which one you choose determines how your story unfolds.