Harry Potter is now in his 5th year at Hogwarts School of Witchcraft and Wizardry, and things just keeping worse. After witnessing Lord Voldemort's return at the end of the last school year Harry finds that the Daily Prophet is slandering him, and that everyone thinks he's crazy. Oh, and did I mention he has a new Defense Against the Dark Arts teacher that rivals Voldemort as the most terrible wizard ever? Through all of this Harry has to deal with his angst and limited contact with his loved ones to arm himself and his friends for the impending battle.
If I was to sum up Order of the Phoenix in one word it would be angst. Previously, Harry's never been one to give into self pity, but he throws that away during his 5th year at Hogwarts. No one understands him, no one believes him, his scar hurts, blah blah blah. I get that he's a teenage boy going through a tough time, but Harry's attitude is the thing that always makes Order of the Phoenix such a trying book for me.
In reality, this is the book where everyone messes up. Fudge won't believe Dumbledore, Dumbledore doesn't trust Harry, Snape can't get over his personal vendetta against all things Marauders and Harry's to obsessed with his dreams to see them for what they really are. That is the thing I like about Order - that no one is perfect. This isn't a children's book anymore. There is death and destruction hiding just blow the surface, and everyone starts to fear for their lives.
While Order of the Phoenix isn't one of my favorite books in the series it's still a masterfully written novel, and one of the most important books of the seven. Also, you don't need to be afraid of the length. I always feel as the books get longer they read so much faster, since things are happening much quicker.