I will not deny that Ms. Rowling has created an intriguing, imaginative fictional world which many people adore, my family among them. I am not one of those people. The serial has been brought up many times this past week at work among my coworkers (one of whom listens to the audiobooks on his iPod instead of actually reading them) and as a result I have formed a more concise reason of why the books just don't work for me.
1.) Dog Latin
My foreign language was Latin, which is dead. I chose it because it was the language of science, and because I have a penchant for Ancient Rome. And a great deal of English words are derived from it, allowing the lay man to both muse and ruse as a scholar of sort. For these reasons it should be granted some flexibility in academic and literary uses where it is far more practical to insert some Latin words than construct an entire language, although it has been done before, and it has an advantage over other languages which do not have any native speakers, like Esperanto. Most often, some words are invented to give the feel of another language, without icky worries of maintaining a grammatical consistency throughout. But please, just do one or the other, at very least be consistent within the pattern you establish for yourself!
Sports are meant to be watched, not read about. No other medium can properly convey the excitement of witnessing the action, preferably firsthand. Yes, I mean you, radio sportscasts. The action is greatly obfuscated by the fact that the sport is both fictional and in 3 dimensions, which no sport, with the possible exception of water polo (which even then is only viewed from two dimensions by the audience), does. Plus the rules are retarded. Basically, catch the snitch to win and, in the mean time, bludgeon each other with these cricket bats. Pay no mind to the fact that the children playing this game wear no protective headgear or safety harnesses and fly at high speeds at altitudes that give muggles vertigo. At very least, I would hope that they would wear some sort of protective cup to shelter their manbits in case those dowels get finicky.
Honorable mention: Adverbs.