
The movie focuses on the romantic relationships that build in the sixth book and the death of a very important character. That's it! They fail to really pull you into the battle that is taking place between the Order of the Phoenix and the Death Eaters. Not sure how they are planning on pulling the seventh book off since that all but left that important battle out of this film.
One more thing...they left out many of the details (memories) about the Half Blood Prince...the memories that you read about in the book give you a much better concept of why Dumbledore even wants Harry know them. It's a pretty important point that is all but skipped over in the movie.