I probbly should na say this, but here goes. (SPOILER ALERT)
What is so great about Harry Potter? I've read all six books now, seen all four movies, and I still can't figure it out.
I'm not saying they suck, because they don't. They are extremely engaging stories. But like the very worst secret agent and superhero films, the plots are for the most part laughably unlikely.
Take the Goblet of Fire, for instance. If you are powerful enough a wizard to defeat and capture an ex-Auror like Moody and take his place, fooling everyone including Dumbledore, why in God's name do you need to go through the whole elaborate Tri-Wizards Tournament and risk the million different ways it can go wrong and you be discovered and your identity revealed, if all you really want to do is kidnap Harry Potter and take him to Voldemort? It's ridiculously complicated. Catch him in Hogsmeaded and teleport him out! Problem solved.
But Goblet of Fire is one of the best of the series! It's a great story, but the plot makes not a lick of sense. That's not supposed to happen!
Now take Chamber of Secrets. Tom Riddle wasn't really after Harry Potter. Jenny Weasley made a much better target. Harry only found the magical diary by accident, and the accident made sense in the plot. The story is much more plausible. And it sucks. It's not the worst of the series, but it is second worse.
The worst is definitely Order of the Phoenix. If ever there was a filler book in a series, that was it. Two chapters at the beginning of Half-Blood Prince would have been sufficient to cover everything in Order of the Phoenix, and it would have spared us from having to watch Harry Potter act like an obnoxious 15-year-old for 500 pages. The whole thing was dreadfully tiresome, and nothing but a set-up for Half-Blood Prince. Again, if all Voldemort needed was for Harry to get the prophecy, why the elaborate set-up? Just do it, man!
So why is it that the series is so good? I have very discriminating tastes in fiction. I recently read two of the giants of spec fic - Ringworld and Ghost Story. I was impressed by neither. I'd say Ghost Story was relatively awful. But Ringworld is considered one of the great all time sci-fi novels, Ghost Story one of the scariest horror novels ever written (whatever!). Not to me, they weren't. So when I say the Harry Potter series is good, I mean it.
But I can't, for the life of me, figure out why they are good.