I didn't read the books.
Prisoner of Azkaban— and to a lesser extent Chamber of Secrets and Goblet of Fire— had a necessary mix of whimsy, humor, color, and charm, and also seriousness/darkness. The series should have stayed that way.
But then first they sucked out the literal color, then the literal light, then the happiness out of the movies. By the time I saw the earliest part of Deathly Hollows, I just stopped caring. Every moment became joyless and hard to watch.
If there's no more discovery of the whimsy of the wizarding world, no more wonderment, no more seeing characters be just plain likeable, why do I care anymore? The series slowly became about friends being jealous and petty, people being double-agents, and about death and loss.
I had to read a summary of the events of Deathly Hollows, because I couldn't subject myself to 4 more hours of misery after the latter two-thirds of Half-Blood Prince. Now I'm just going to enjoy the full LEGO Harry Potter collection game.
I loved how the movies gradually changed from kiddy comedy to borderline horror. I'm more into the latter style so I'm okay with it but I can understand why it may be disappointing for those who fell in love with the former.
Thank you for an understanding response. I myself do not like a focus on horror, cynicism, bleakness, or grit. Of course that's not black-and-white— I can appreciate scary suspense too.