I love re-reading books. You always read it in a different way and you always remember things you already forgot.
1. Ron and Hermione were made prefects
2. Harry and the twins were banned from Quidditch for life
3. Luna's first appearance (Luna is such a part of the Harry Potter universe that I totally forgot that she doesn't appear until the fifth book)
Things you will never forget:
1. Dolores Umbridge, the most hated charakter in the world
Now I don't want to read the last two books.