“Although it is often regarded as the weakest installment in the "Harry Potter"-series, I regard OotP as more of a "flawed gem". Harry is finally behaving like a teenaged boy ravaged by raging hormones (irrational, irate, belligerent), and even though [spoiler alert] it is later revealed that this behaviour is largely owed to the sinister connection between him and the Dark Lord, I find that this particular aspect is one of the novel's great strengths, emphasizing the way the protagonists actually grow up throughout the series, instead of being static characters.
What I *dis*like about the "Order of the Phoenix" is the way J.K. Rowling allowed herself to be influenced by the fans, and/or deliberately wrote her book with certain teasing announcements in mind.
After the tantalizing reveal that somebody was going to die in this novel, it becomes only too obvious that every scene featuring mortal danger has been deliberately geared towards playing with the expectations engineered by this.
You could also criticize that this novel does very little to actively advance the overarching plot, as the status quo does not change that much over the course of the whole novel, barring the public acknowledgment of Voldemorts return in the end.
All in all, this novel might not be the best in the series, but neither is it the worst.”