For me it's just the small details that stick with you. Like Harry, Ron and Hermione sitting on the train on their way to Hogwarts, Lupin offering them chocolate after the Dementor scene, Harry wearing Dudley's old over sized t-shirts, Harry meeting Cornelius Fudge after running away from the Dursley's, Dumbledore's love for socks and sherbet lemon, Miss Figgs showing Harry pictures of all the cats she used to have, Mrs Weasley's kitchen, the garden gnomes in the Weasley's garden, Hagrid's awful cooking, broken Viktor Krum doll under Ron's bed, Ron mourning Scabber's death and blaming Hermione and her cat for it, Slughorn serving cold pheasant to his favorites, Harry's teammates bringing the broken pieces of his Nimbus 2000 to his hospital bedside,... I could go on and on.