I preordered the movie and have watched it twice since, so heres My take on the 3rd movie:
I found it interesting that they could cut so much out, yet left in all these bits that noone would understand, unless they had read the book. The best example I can think of, is Lupin's prior knowledge of the map and how to use it. He knows the spells to start it and turn it off, yet the movie does not detail that he was part of the making of the map.
Another example which they really should have included, was how Sirius knew that Petigrew was at hogwarts, and how Sirius would talk in his sleep saying something like "He's at hogwarts" (after seeing scabbers in the picture of the weeslys in the paper) which made them think he was after Harry. Also, they should have shown when Sirius went to attack scabbers, which scarred the shit out of Ron in bed. That would have been halarious and would've added to the mystery of Black. The lack of this scene also made the attack on the fat lady, seem really random.
Something I didn't like about it, was that Harry's patronis was not portrayed properly (in my oppinion). After Harry goes back in time and watches himself at the lake, I remember the book describing his attempts to cast the patronis as 'pathetic' (or something to the effect), yet it was still pretty cool and amazing to watch. I was expecting sparks, then nothing, making the final successful patronis to be VERY amazing in comparison. Also, the stag was hardly distinguishable, which sucked.
Another complaint is the portrayal of Draco. I expect Draco to be strong, so that he is an excellent nemesis of Harry. Yet, twice in the movie, the audience sees him wimpering. And, he gets his arse kicked by a girl.
Apart from these, I REALLY loved the movie.