First off, Lina Leandersson is fucking amazing. She gives a very nuanced performance, showing wisdom far, far beyond her age. I don't even know where this girl is mining her emotions from, but it works brilliantly.
She literally has the world-weariness of someone several thousand years old.
I agree with franco that restraint is key here. Both narrative and visual. The viewer doesn't quite know the entirety of what's going on until the second act, but the mood of dread is established early and held. What Alfredson chooses to show or not to show (in his frequently wide angles) is key. He never releases the tension until the audience will get the full cathartic effect.
A particularly Hitchcockian moment is when Oskar tries to make a blood brother without knowing the full ramifications of his actions. As soon as he takes out his knife, the viewer knows what is going to happen, but the characters don't. Alfredson consistently uses classical storytelling and suspense like a master of cinema.
On the point of a guardian angel and the visual restraint...
My personal favorite moment in the film is when Oskar saves Eli, and the roles are reversed. The young boy is unable to kill, as he thought he might. And when Eli is awakened and leaps onto the intruder, Oskar just lifts his hand and shuts the door. This is a great inversion of the famous shot at the end of The Godfather. Of course, in this case, Oskar is shutting in, instead of shutting out. When Eli emerges, bloody and victorious, Alfredson again shows such restraint, allowing just a hug. Also, keeping in mind the theatrical lessons of the line of sight, he keeps Oskar's eyes off of Eli, constantly suspending full resolution, full catharsis. It's masterful storytelling.
Let The Right One In is a precise example of how to reinvigorate a genre. The editing rhythms and the selective emphasis in the sound design bring a musicality and flow to the piece. The score follows the currents, but never tries to change them.
For discussion, I'd also like to offer a more unsettling reading of the film:
Removing the vampire mythology from the film, what you have is essentially a revenge fantasy without making Oskar culpable. In a post-Columbine school environment, Oskar is able to behead and dismember his enemies and escape unscathed.
Of course, in the telling of the story, Oskar doesn't do any of these things, and Eli even spares the one child who backs off of Oskar in the pool. So aside from Oskar being innocent, even the one concerned child was spared. But how does Eli know that the concerned child backed off of Oskar? Was she there the entire time? If so, why did she wait so long for the rescue?
What if she was Oskar? There is certainly a duality to the personae of Oskar/Eli. As evidence, I'd first offer up the scene where Eli tells Oskar she wishes he could be her, at which point Alfredson cuts to a close-up of an old woman, presumably Eli's true age. The cut is jarring, and after that sequence, the moment is seemingly forgotten.
Furthermore, nobody really even knows about Eli, except when she strikes. Of course, Oskar knows about her. And lest we be diverted by gender confusion, Alfredson (or the screenwriter) simply removes the issue from Eli.
I'm not suggesting that the film is Fight Club. I am saying that there is a certainly a reading of the film that Alfredson offers the psychoanalytical "lashing out" and cathartic violence of a school massacre without offering culpability to Oskar. Especially since only the bad guys got hurt, and Eli only kills because she "has to."