It's hard to diss Harry Potter - it's an incredible achievement that has completely changed the style and tone of much juvenile fiction - but I thought the last three books failed to live up to the promise set by the first four.
I think it collapsed under the weight of its own concepts a bit. By the end, JK Rowling had maybe introduced a few too many completely original ideas that seemed a bit... arbitrary? What I mean is, it's easy to feel emotionally involved if the kids go up against a troll or a giant spider cos these are familiar concepts. But once the plot got bogged down in Hallows and Horcruxes and Patronuses it felt a bit too abstract for me. Maybe that's just me.
The final confrontation with Voldemort was a big let down for me. I won't say more in case of spoilers.
But regardless of the things I would have liked JK Rowling to do differently, the whole series is, on average, brilliant. The fact that I was slightly disappointed by the last three books is more a comment on how good the first four were.
Also, I don't think JK Rowling is a totally fantastic writer of descriptive prose. I find her prose style a tiny bit plain. It does the job, but never made me stop and think, "Wow, what a beautiful passage." However, I think she must write world-beating dialogue or something, because all of her characters seemed so real to me. Part of the joy was just sitting back and reading while Harry, Ron and Hermione had their little rows and disagreements about the most mundane things