Finished Harry Potter and the Deathly Harrows a while ago, but only now found the time to write it up. Essentially spoiler-free, but in the comments area, anything goes:
Here, in the series' final book, is where Rowling strives most evidently for long-term grandiosity, from the Pullman-esque epigraphs, to the honest-to-God old-school Fantasy Quest, to the (disappointing) abandonment of "school" as the primary framing device. She also takes this as an opportunity to effectively trash the franchise, attempting with unrestrained relish to definitively retire most of the major characters (in one fashion or another). Some of the sacrifices thusly endured would feel (more?) capricious if it weren't for Rowling's selection of Life Under Enemy Occupation as the replacement frame. As anyone glancingly familiar with the history of WW-II-era Europe can tell you, enemy occupation makes for harrowing circumstances, and it is these circumstances that the book, at its best, convincingly evokes: no place is safe, everyone is constantly at risk, ignoble death can strike seemingly at random. This is a dark place for the series to go, but it sets the stage for satisfying closure.
Over and over again during my read of the series, I thought about the act of world-building, and how it is distinct from or related to the more traditional acts of narrative construction. Expect a discussion on this point soon, using the Potter books (and possibly the Pirates of the Carribean series) as the prime exhibits.