The poor assortment of plots compared to what should actually be important, not to mention the boringness of most of the films, the lack of spectacle(which is the whole point of the franchise), and my lack of care for the main characters.
Personally, I dislike the entire series as a whole, not to mention I didn't even like the books.
My personal grievance with HBP was the lack of Voldemorts backstory.
I was hoping that since the final movie length was 2 1/2 hours, they had only removed some scenes and would come out with an "Extended Edition", but this seems not to be the case.
Our "brilliant" director wanted the movie to focus on Voldemort's current rise to power, and not get overly involved in his past. Personally, I think Tom Riddles original rise into Voldemort is more engaging, and interesting than his second rise to power [just saying]. Plus, it explains to us WHY he is who he is, and does what he does. We come out with a greater understanding of him, and that [potentially] makes him even more frightening than he is. Plus, with this movie focusing on the development and coming of age for the current generation, it would have been a nice counterpoint seeing Voldermort's coming of age story.
I think the scenes involving Voldemorts past could have easily been put in without disrupting the flow of the movie, and in fact would have enhanced it. Hopefully, some of his past is revealed in the remaining two movies since that is plenty of time to do such things.
edit: Though I have to admit some of his choices were good, don't get me wrong, it's just that the Voldemort parts didn't have to be axed.