The Harry Potter books are very well-constructed. There are things from the very beginning of the story that unexpectedly become important all the way at the end. In terms of how to write a long saga and keep all the parts of it connected - without giving away the connections to the reader early - Rowling put on a damn clinic. It's an astoundingly well-built story.
However, this doesn't really become clear until the last couple of books, when all those setups from the early novels start coming to fruition. I can see how someone who only reads books 1-4 would think they're not good. You have to read the whole series to get the payoff.
As for language: Rowling became a much better writer as she completed the books. The last 3 books in particular are miles better than the first 4 ... not just in her writing, but in the realism and life she brings to her characters. If someone opines that her writing is still bad, fine - but she sure as hell tried to get better, and as far as I'm concerned she did it.
Just a couple of weeks ago I re-read books 6 and 7, and both of those are GREAT books. Anyone who has avoided this series because of hype, or because it started as a children's series, has only deprived themselves of a superb story.
Of course LotR and Potter are similar. They're both drawing from the standard elements of long epic stories that go all the way back to Gilgamesh. Criticizing either because they contain something like a Dark Lord or old man advisor is silly. Those things are part of the genre and are almost impossible to skip. It's like criticizing a heavy metal song for containing a guitar riff ... it's not only there, but it's expected, and avoiding it ruins the story.
And for those of you who say you "grew up" with Tolkien ... maybe, but I'd put my Tolkien fan creds up against any of you. I first read Tolkien over 30 years ago. My Silmarillion is first edition. And I had to walk twelve miles to the bookstore, uphill both ways, through 3 feet of snow to get it! </grumpy old man>