I'm thinking notably of the train scene when Harry - and Ginny - are the only ones to be particularly heavily affected by the Dementor. Is it exaggerated to see that as the beginnings of what we'll get to see full-blown in book five - Harry arguing that they don't understand him, they don't know what it's like, they haven't had his experience with death, be it through his parents' death or his own close encounters with it.

The sixth chapter also does a lot to develop Hagrid's character, I thought. It's interesting to see how he acts as he thinks a teacher should act, though obviously still with the blind spots about magical animals.