Thanks ShaniFaye, I had totally forgotten about the argument! Time for a bit of re-read and pondering.
Now that I have had time to think things over a bit, I've found some more interesting points. First of all: Spoiler:
The Sorting Hat. The four founders have all "put some brains in me" as the Hat says in GoF. Is it in fact a Horcrux?
Also, note the beautiful mirror symmetry of polyjuice usage. In the second book, Crabbe and Goyle were impersonated by polyjuice, in the second-to-last book Crabbe and Goyle are using polyjuice (to impersonate girls, the slash writers are going to have fun with that!) and polyjuice played an important role in the middle book too. I hope there will be no polyjuice in the last book, to make it perfect.
Best sentence of the book:Spoiler:
"Harry, however, had never been less interested in Quidditch; he was rapidly becoming obsessed with Draco Malfoy." 