Fantasy and sci-fi need to be grounded by rules. Precisely because characters in these genres have magical powers and encounter mythical creatures, their abilities need to have clearly defined constraints. We don't want to feel cheated when characters arbitrarily have new and never-before-seen powers to extricate them from plot problems.
Perhaps the most famous example of this kind of cheating is the movie Superman II, in which Superman had a magic kiss of forgetting. However, Laurell K. Hamilton's popular Anita Blake series has also been criticized for regularly giving its protagonist new superpowers to get her out of jams. This stuff isn't good; it's difficult for readers to invest in conflict if they know that the character is probably just going to solve any plot problems that arise by shooting heretofore-unmentioned laserbeams from her fingertips or blowing stuff up with her mind.
"Damn right, I have a magic kiss of forgetting. I'm friggin' Superman."
We call this deus ex machina, which is Latin for "God from the machine." The term refers to a convention of classical theater, in which plot problems were resolved by the sudden entrance of a deity into the story. However, in practice, it refers to any solution to a plot problem that emerges from outside the narrative. It is, in some ways the inverse of the well-worn rule about Chekov's gun; the rule that a gun hanging above the mantel in Act 1 must go off in Act 3. It is similarly true that, if you want the character to be packing heat in Act 3, you should establish the gun in Act 1.
Grounding something organically in plot is structurally simple; James Bond swings by the Q-Branch lab at the beginning of the movie, and Q demonstrates some new exploding cufflinks. Later on, when the villain thinks Bond is disarmed, we realized that Bond is still wearing the cufflinks. Bond will then proceed to use the cufflinks blow shit up. We understand where the cufflinks came from, so this is logical.
Compare and contrast: There are no cufflinks. When Bond is captured, he suddenly announces he has magic powers and starts blowing things up using psychokinesis or something. This is deus ex machina.
Vampire fiction has a long and dubious history of bending its own rules and loopholing its central premises. Vampires are sexy and readers love them, but their pesky problems with daylight and their inconvenient need to drink blood all the time makes it hard to write plots about how they are glamorous bad-boy romantic heroes with awesome superpowers and no drawbacks at all, whatsoever.
Twilight gets bashed a lot for its blasphemous sparkles, but beloved Buffy the Vampire Slayer struggled with similar problems, and the Buffy writers were fond of ridiculous, illogical solutions. Vampires could buy blood at the supermarket in takeout soup containers. Every building in Sunnydale was connected by convenient, easily-accessed sewers, so vampires could get around during the day. Failing that, they could run around with blankets over their heads.
"I could never love you, Spike. You smell like doody."
After the jump, more spoilery stuff about Justin Cronin's The Passage.