It's hard to describe exactly what it is about the books after The Prisoner of Azkaban that I didn't like. I read them, wanted to read them, just to find out what happened. Perhaps my issues with the book are not major flaws, but I found them to detract from the books.

First of all, Harry Potter and his friends have got to be the most pathetic magic users ever trained. Even Hermoine, who is presented as the smartest student of the school, is, really, quite pathetic. It seems like everyone in the previous generation was much more skilled by the time they graduated, or even before they graduated, Hogwarts. James and company were skilled/powerful enough to make the Marauder's Map, a feat I doubt Hermoine (lets not even mention Harry here) could replicate if her life depended on it. Tom Riddle created a copy of his memories in a journal while still, what, a sixth year? Even Snape was a capable potion maker (to the point of improving the recipes) and seemed to either know an advanced curse, or created it as recorded by his memories and his journal. But the students in the present? They are all failures. Not one of them is creative, they just learn what was previously discovered and hope, at best, to replicate those past successes. None of them improve anything, none of them create something that did not exist before. Even when Hermoine made the coins for the DA, she used a spell she found.

Also, for a world where magical items seem to be rather important, Harry Potter and friends seem to know remarkably little about making such items. The best they can do, or rather Hermoine can do, is make a coin grow warm. Another important skill that none of them show any sign of knowing is making their own spells. I am sure Voldemort didn't just use a spell someone else made to make his horcruxes or the bowl with liquid that kills you with thirst. He made them. Likewise, Voldemort makes a silver hand for Peter (not even sure if he said any words), the spell that makes the dark mark, and almost every single time Dumbledore uses magic. Related to this is the stream of charms that Flitwick was able to use to defend Hogwarts when it was attacked, granted it seemed like no one but Flitwick was able to do this.

I find it ludicrous that Harry was able to fight off trained killers using the disarming spell almost to exclusion. It defies logic. A spell they learned in the second year of Hogwarts remained effective against the most dangerous wizards, until all of a sudden, it didn't. Of course, I refer to the moment that countering spells was taught in Hogwarts (this is related to my second complaint, the fact that nothing exists until it has been taught in Hogwarts). Then, all of a sudden the same tactics that would have been successful in earlier books, surprise spells and such, fail. Why? Cause the Professors (and enemies) have suddenly become amazingly good at countering spells.

In short, Hogwarts seems to be a terrible place to learn magic. This is even more disturbing when you realize that there is no "college" for them.

Second, as I stated before, things just get added to the world. I suspect this is because Rowling didn't quite think out the kinds of magic she would allow in her world, and just added it, without thinking of what the consequences of adding it should be to her world. These books are definitely no Earthsea. The first symptom of this is that many of the books had the "spell of the year." In the second book - disarming, in the third - patronus, and in the fourth, stunning and the three big ones. These spells are remarkably powerful when introduced, in fact grown wizards are depicted using them in professional work, and yet in previous books they are not used, sometimes by those same wizards. No one disarms anyone in the first book. No one stuns anyone before the fourth book, even though the professors, at the very least, should know it (particularly annoying when it's shown stunning a dragon, stunning evil wizards, **** stunning everyone in that book). Similar to this is the counter-spelling, suddenly introduced, and suddenly the Professors are proficient.

And of course, the killing spell. It's unblockable! Until it's revealed that it's not. No, all you need is a bit of older magic, some sacrifice, and it can be blocked. By magic. But then, there might be other ways to block it. It's just harder to block than other spells (not that this even means anything, almost all the spells seem to take instantaneous effect if they hit you and you can't do a damn thing about it). The best that can be said is that you seem unable to erect a shield that would block it, like you can with "minor" hexes/curses.
Another thing that was randomly added was the a spell that, amazingly, made you unable to betray someone! It would cost you your life. You know what, I wonder why Voldemort didn't make use of this spell. Did he think his people were trustworthy? Would that even matter to him? Also, combined with the secret keeper magic, it seems to be an unbeatable combo. Frankly, once that was introduced, I felt that James and Lily died for no reason whatsoever.

Not to mention that a world where truth serum exists should not be imprisoning people falsely. There is no way Sirius should have been imprisoned. It defies belief. But of course, in the third book truth serums did not exist because they had not been introduced yet. No, had to wait until the fourth book for that.

Portkeys were also added randomly. They could also be used to move in and out of Hogwarts, which was supposed to be impossible. But, I kind of gave up on Hogwarts security when the disappearing closet could be used to get in.

Third, just random inconsistencies in the universe. For example, polyjuice potion is remarkably overpowered. Combined with the spell that controls people, I find it remarkable that any government is possible at all. Or that wizards can trust anyone. Then there is question: given the existence of an ability to counter spells, why use spells that affect a single person? Peter blew up a whole street when he faked his death and, by all accounts, he was not a powerful wizard. More AOEs, less single target spells, would seem to be called for, and yet, what do the fights between the death eaters and the order look like? A **** **** storm of single target spells. Only Crabbe, Goyle, or whoever, seems to get it right when they make that fire that burns everything, and even then they can't control it properly.

Also, who made the defenses of Hogwarts? Who made the room of requirement? What kind of magic is involved in that? Who makes anything? It seemed like the whole book the only people who make things, who show up in the present, are Voldemort, Dumbledore, and the twins! Brooms, books that eat people, invisibility cloaks, the enemy detector, all of these are made (with the exception of the invisibility cloak) in mass numbers, and yet no mention is made as to who made them, nor do the students seem to be learning any skills that could be related to the creation of such items.

tl;dr: Rowling showed absolutely no planning of the universe past the third book and added things as she liked. Also, Hogwarts is a failure of a school, and Harry and his friends are terrible magic users.

Edit: Omg! I forgot something. Why would anyone who is capable of using the killing spell, use any other spell in a life or death situation? Why? When there is no "power" or "mana" or anything that they lose. In fact, no one in the Harry Potter universe seems to become tired at all when using magic, it seems to cost them nothing. Given this, why would you use anything but old faithful? Do they like handicapping themselves? Do they like seeing their spells blocked? Do they like failing? This is, of course, the only spell I can see being used other than AOEs.

The books were written for kids. The fact that there aren't sparkly vampires in it adds to it's credibility and offsets any inconsistency it may display.

If there were sparkly witches in Harry Potter I'd ridicule them too.

It never seemed to me that Hogwarts was really that concerned about the safety of it's student body...

Then there is question...why use spells that affect a single person? Peter blew up a whole street when he faked his death and, by all accounts, he was not a powerful wizard. More AOEs, less single target spells, would seem to be called for, and yet, what do the fights between the death eaters and the order look like? A **** **** storm of single target spells.

They are, however, not nearly impressive enough to warrant JKR's billions. I don't get the widespread collective fanaticism.