The Sixth Book is when motivations become clear, alliances are broken, and the risks are higher. Two VERY important plot points happen in this book. One is a huge death toward the end of the book, which I won't spoil for new readers. And the other is the background on Tom Riddle and how he came to be Lord Voldemort. You learn about his birth mother and her terrible family, you learn about Tom Riddle, Sr. and his family, and you learn how Voldemort used Dark Magic to extend his life. All very exciting and new stuff happen in this novel.
I didn't really feel the length of this book unlike OOTP. Very good read. Obviously if you have gotten this far in the series you would undoubtedly read this novel, so don't delay and finish this book!