I think it's cool how JKR brings things back from other books. Like the Vanishing Cabinet: Remember how in book2, Nearly-Headless Nick Crashed above Filch's office to get Harry outta trouble (Chapter 8: The Deathday Party). The Hand of Glory Malfoy was goggling over in Borgin & Burkes. Riddle's diary being a Horcrux. The image in the Erised mirror not really being Harry missing his family, but the weapon to kill Voldemort. Can't remember anything else, yet...what else is there? There's more, I know...