The worlds J.K.Rowling created have some similarity to other fantasy books. Most creative people "stand on the shoulders of giants", this doesn't detract from their originality.
One of the fathers of modern fantasy was Tolkien; creatures from his books are used by most in the genra. He was inspired by Icelandic sagas.
Like Tolkien Rowling used trolls, goblins, dragons, giant spiders. The first incident that makes Harry Ron and Hermiony into friends is fighting a troll. Not unlike the hobbit.
Frodo and Harry are orphans. Raised by relatives but in very different circumstances.
To kill sauron you need first destroy the ring. Like voldy with his hoarcruxs.
Carrying the ring makes the person feel low, like the hoarcruxs.
Sauron is referred to as "he whom we do not name" by Faramir of Gondor. Others say "the nameless one". Voldy is "he who must not be named".
Gandalf and Dumbledor. Both are mentors to the lead character, both die but come back after a fashion. Both speak the name that others fear, "the nameless one", or "he who must not be named".
All this takes nothing away from J K Rowling, her books have much originality. They are best sellers and got children turning pages in a digital age.
Some other influences:
The weasleys have some similarity to the borrowers, red hair, close nit family who live in a burrow? Of course the burrow is above ground.
Also "The Worst Witch" books and tv series by Jill Murphy has a school for child whitches. This episode recalls Harry's first time on a broom. http://youtu.be/jjkwoD-QXms
Terry Pratchett"a unseen university may also have been influential.