Similarities to Stephen King's It
I've noticed a lot of similarities to IT:
Both bullies (Kenny and Henry Bowers) are psychopaths
Both Kenny and Henry leave a wound or mark on a kid's chest or face
Both Kenny and Henry are attacked by the kid/kids they bullied
Both Kenny and Henry almost kill a kid, until they are killed or traumatized by the demon
Both feature a gang of bullies getting brutally murdered by the demon
The vampire is female, but actually male, while the clown is male, but actually female
Both use a nostalgic decade to feature kids' childhoods (It; 50s, Let Me In; 80s)
Both films are based off a novel
However, Let Me In doesn't include the adulthood aspect, unlike IT.