I noticed a few things I hadn't before: Tonks' hair color changes from purple to red to blue when Moody introduces her as "Nymphadora." Kreacher was hiding something when Harry sees him in the hallway at Grimmauld place the first time.
Harry teaches the DA "Levicorpus" and Luna uses it on a Death Eater at the Ministry. In the books, Harry doesn't learn that spell until Half Blood Prince.
Devon Murray (Seamus) is really short. Bonnie Wright was standing beside him at the end and seemed much taller.