My DVD came yesterday, so I got to see HBP for the first time. It doesn't give me any pleasure to be the voice of crabby criticism, but, oh dear . . .
Clearly it would have been better if it had been Remus Lupin and the Half-Blood Prince. Some adjustments to the plot of the book would have been necessary, but well worth it.
To me the meat of the book was the backstory on Tom Riddle that ties up the loose ends of his descent from Slytherin and how he wound up in an orphanage. And it sets up the quest for the horcruxes. This was slashed to a minimum in the film. By the same token, the teen-wizards-in-love action in the book was filler because, honestly, Harry is the only one with anything significant to do--pensieve sessions with Dumbledore and getting that memory from Slughorn. The filmmakers wanted to keep Hermione and Ron and Harry on screen, so they put the teen-love filler and quidditch fluff center stage and even added to it.
The burning of the Burrow had nothing to do with anything, except it provided a scene for characters who would otherwise not have had a scene to be in--except, no! wait! in the book there was a battle in the hallway at the foot of the astronomy tower stairs that the filmmakers cut completely. That had Lupin in it with Tonks, and they were being heroic and dashing instead of running around in tall grass and standing in a puddle. What a waste. And since we lost Fleur and Bill, we lost the Lupin and Tonks arguing about her being in love with him and Lupin giving in too. Drat.
The whole question of who the Half-Blood Prince might have been got dropped entirely. So when Snape stalks up to Harry and identifies himself as the HBP, there's no juice in it; it's as if the line is there so there's some excuse for the film's title.
There were some good touches here and there. I liked the upraised wands and their light blowing away the Dark Mark. That was more moving to me than the protracted funeral in the book. The aggressive popping of the photographers' lights when Harry has just shaken off Voldemort was excellent. And the scene in the diner was a welcome improvement on the eternal Dursleys, and did far more to fill out Harry's character. But the little things weren't enough when so many larger opportunities were missed.