Some things - the dry wit, the careful plotting full of red herrings and subtle clues, the subtle or less subtle social/political criticisms between the lines - were there already, though this book does contain some of the best examples so far of all of those things. But as the Trio has now fully hit puberty, she proves herself as a decent writer of teenagers, portraying them realistically but without alienating her pre-puberty readers or the adult ones. It's not on an Aidan Chambers level, of course, but then their feelings and development aren't really central to the series as they are to Chambers' books. For doing it as just one among many elements and themes in the series, I think she does it very well. At the same time she has the Trio (or at least Harry and to a lesser extent Hermione) really becoming players in the adult world even if they aren't adults yet, getting really involved in adult matters and politics.

And then of course the themes that were already present in earlier books become more explicit and more complex, as the series matures. Abuse of power in all its forms, racism, people distancing themselves from their family or their environment (for better or for worse), judging people too soon without knowing all the info, the power of the press, group pressure and mob mentality, the dangers of bureaucracy, and there are probably a few more that I'm forgetting. That's really not a bad list of themes to tackle in a children's series, I'd say. Though from here on out you can debate how much it's still a children's series and how much it's really YA. As in, actually YA - most so-called YA tends to be for children aged 12 or so, but the later books in the series are more written for, well, children the same age as the Trio, I guess.