From The Atlantic's review of DH by Ross Douthat:
(Rowling's) genius is for crossing genres, mixing the sword-and-sorcery tale with the boarding-school novel, and merging both with the intricate plotting of an Agatha Christie story. And this is the first problem with her saga’s grand finale – it’s more of a straightforward high-fantasy novel than any of its predecessors, and Rowling isn’t quite up to the task. She needs the Stover at Yale structure, it turns out, and the Ten Little Indians plot mechanisms; without the “something fishy at Hogwarts” framework, her story sprawls and meanders through a baggy quest narrative. The more time Harry and Ron and Hermione spend hopscotching around England, the more the reader misses the familiar rhythms of a school year at the wizarding academy...
I'm more enthusiastic about Rowling's work than this reviewer is overall (he's liked the series but feels the last book compromises the series' ability to hold together as a classic-to-be), but he describes a lot of my dissatisfactions with DH.
But! I am on my second go-through of it, via audiobook, and I'll see how it feels to me this time through.