I haven't read any Harry Potter books, but I've followed the plot via the movies and discussions. In light of how the final book turns out, I've come up with the following statement:
If you create a world of fiction, set out the rules of that world, and then have the plot turn on a suspension of those rules, then that is a terrible work of fiction. Likewise if you purposefully obfuscate the rules of that world and then only reveal them at the last minute, that is bad form.
Yeah, I wait
For something good, for something great