This book is seriously in need of an edit.
I re-read the first four Harry Potter books endlessly when I was younger, but as far as I remember I've only read the last three once each. One of the reasons, I suspect, is that Order of the Phoenix suddenly gets extremely dour (which is not the same as serious). The earlier books are actually quite slyly funny; Order gives up those textual games to present us with a Real Teenager: Harry here is perennially grumpy, constantly shouting at his friends and generally just rubbish in every way.
Of course, he has good reasons to be grumpy: Order is the book where Hogwarts is effectively taken over by Umbridge, the representative of a hostile Ministry for Magic, which is desperate to suppress talk of Voldemort's return. But it's not really much fun to read about, partly because Rowling just goes on and on and the taut plotting of the earlier books is lost in all that padding. There's a lot here which feels like it should be more significant than it ends up being: the Quibbler interview, Harry's romantic experimentation, Firenze's stint at Hogwarts. In the early books, all of this would have been satisfyingly and surprisingly tied to the ending. Here, it's just - stuff, self-indulgent and not really interesting enough to justify the page time spent on it (Order is 700+ pages).
I know there are thematic reasons for the eruption of teenage angst here - as Voldemort returns, the power of children to defeat the evils in the world fades - but Rowling isn't a good enough stylist to let thematic reasons carry her book.