OOTP is a dark story. Like Harry I too spent much of the book angry at what was happening. I really liked the changes to Neville, there are hints that he may have a larger role in the later books.
Almost everything changes for Harry, and the others, in this book. So many people have to deal with losses. Especially the Weasleys. More than one person dies in this book and more than one person comes close to dying. Some even get severely injured. There were 3 or 4 times when I thought 'This is it, this one dies.' When it finally came it was without much warning, sudden, and shocking.
The new DADA teacher is truly horrible. I pictured several different people playing her as I read her scenes. Each one uglier and more repulsively made up than the last. Finally a teacher at Hogwarts that really makes you realise what a nice person Severus Snape really is. I also enjoyed the fact that all except one of the former DADA teachers were in this book.