I think one of my more favorite things about good-old Harry Potter is that bad things happen.
Sure, the good wins in the end, as it should, but bad things happen on the way. To me, that's fairly realistic. My belief with fantasy is that it needs to be grounded in some way, otherwise it's just a crock. Without the bad, there is no good, right? Plus, without the trials, the really good parts of the novels wouldn't be as satisfying.
The kids get detentions and are punished for not following the rules (even though they are working against the rules to save people, but still, you've heard the term "no good deed goes unpunished..." right?).
You've got heinous Draco Malfoy and Professor Snape. They are both awful people, and you wish you could just slap them both, but you can't necessarily slap a teacher, right?
Okay, so Potter blasted Snape back toward the end of "Prisoner of Azkaban," but the dude had it coming.
Still, though. They're always getting in the way, and you know what? There are people out there like that. Especially Draco. Snot-nosed kids like that just plain suck, especially in real life.
Going back to "bad things happen," I liked how the bad guy got away in the end of Book 3. Oops, I spoiled it, didn't I? Not really. If you haven't read the series, then you've seen the movies. If you've stayed away from both, you probably will continue to stay away. If you did plan on reading it, and you think I spoiled it, eh, well, maybe I did, but not really.
Anways, I wasn't satisfied with the ending (well, I was, but you get my point). But, that's when you realize you're emotionally involved with the story when you get frustrated about things like that. Plus, that would be pretty sad for the writer if she tied up every story with a neat bow every time. If the writer isn't making the reader mad at times, she's not doing her job.
(I personally love to take the characters in stories where the reader may not want them to go)
I also liked how Rowling showcases the bureaucracy of education in the books. What? I know, weird, but there seems to be some parallels to how schools are fun (kind of). We have the Lucius Malfoy, who seems to have great pull in some decision making, and also has much influence over some of the decisions (or tries to), even though he has no idea about teaching. Sounds like public education to me!
Now, I begin Book 4. I like how she changed things up in this one and started with Voldemort. Nice touch.