the peverells. they originally had the wand, cloak and stone that made up the deathly hallows and it's important for the audience to know that voldy is a direct descendent
and dumbledore explaining about the charm on number 4 privet drive that will break as soon as harry turns 17. important because there's the escape sequence at the start of dh
why wasn't harry punished for using sectumsempra curse on malfoy?
no mention of slytherins locket, hufflepuRAB cup, ravenclaws diadem or anything else that could have been a horcrux. my friend who hadn't read the book had no idea why there was a locket in the basin
the crap battle at the end. harry would not just stand and let snape go near dumbledore if he knew dumby couldnt defend himself. he just wouldnt. he would have gone and fought him
hermiones character went a bit wrong too i think. ok, she was upset about ron and lavender, but crying all over harry? no. one of the themes of the 7th book is wondering if they'll end up together because neither will admit they like the other
and, although it's not important, jk rowling never actually said that malfoy had a dark mark. it was pretty obvious he had one, but she never actually wrote that he did. i think thats important because he turned out to be good in the end (sort of)