This movie was up to it's usual standards, so HP fans will love it.
It's been awhile since I read the book, so I'm not sure if the explanation was in this one or the last, but the story is about HP and THE HALF BLOOD PRINCE. If you didn't pay attention in the 5 seconds that Snape mentioned to Harry that he was the prince, then you missed the whole thing. There was no explanation of why he called himself that, which is the whole key to the half-blood prince part.
The ending was very weak and they left out critical information. Since it has been announced that the final book will be split into two movies, I expect I'm not the only one who feels this way.