I wouldn’t say that I was disappointed. It was as thrilling, and as interesting as any other Harry Potter Book. Though In this book JK rowling lays down the story to a darker road.
As from Harry’s point of view, we find more rage and a powerful sense of rebellion in the story as Harry reaches adolescence. The story becomes kind of moody.
There are several moments when we feel disappointed, and infuriated like when Harry is stuck in Privet Drive for 2 months with no one in touch, the scene of Harry’s detention with Umbridge, when Harry is banned from playing Quidditch, and when Umbridge becomes the Headmistress of Hogwarts.
But there are certain happy moments which balance the story, like the DA sessions among students, When Dumbledore saves Harry from getting expelled, and the scene Fred and George create when Umbridge becomes the headmistress.
Overall, I think the story was pretty good, only it becomes kind of depressing at last on the death of Sirius Black. Also, the story is pretty long and could have been shortened which JK Rowling admits herself.
Hence It is my least favorite Harry Potter Book, but still one of my top favorite novels.