So her prose still reads like it was written by Voldemort's snake but good grief can J.K. Rowling tell a story. Well-paced, with long episodes in one place broken up by humour and action, and with plenty of vividly-told sequences, it was certainly more enjoyable than the last couple. Its resolution is predictable, its theology simplistic, its characters still quite thinly drawn (I'd never believe Harry experiences any emotion ever), but I was up till 3am finishing it.
There are two things which stand out for me. The first is how Rowling has created a very potent world with a Nazi state at its centre. She does this through scenes such as that in the Ministry of Magic, with its ubermensch accoutrements and cruel inquisitions into blood-purity (indeed, Mudblood crops up throughout) but also through slight touches. You get the sense, through reported speech mainly, of how the rest of the world beyond our heroes is faring, and this gives it an insidiousness which truly unsettles.
The other aspect which works so well is how precisely and tightly imagined the plot of the whole series is. I won't reveal all the details (tho' I would have thought that polar bears had their copies from Borders Arctic Circle already), but either Rowling had countless tiny plotlines laid out before she started, or she's been exceedingly fortunate in tying together loose ends.
Did she know Ollivander would be so important in HP7? Was Riddle's diary always a Horcrux, or did the concept of Horcruxes come later? Rowling is on record as saying she had plotted them all already, but just to have such resolutions is testament to a mind with a flair for fantastic plotting. It also makes her world seem much more coherent and credible.
The funniest riff on Harry Potter was Miles Kington's in the Independent: Voldemort became an airline whose passengers never returned. (Pense en francais!)