As she went on in her series, J.K. Rowling packed a lot of plot into her books...too much to have everything make the cut. Along with that, to make the movies fluid, the screenwriter would have to write fillers to keep the main plot moving...especially with so many subplots cut.
With that said, I think that they did a good job. The set and cinematography were outstanding. The acting has come a long way for a group that started out as small children, and have grown into their young adulthood. Their talent has definately grown over the years.
So sit back and enjoy it not as a stand along (for none of the HP movies stand alone in the series) but as a bridge to move the plot towards the action of the final installments.