I like how the entrance to Hogsmeade (or the Shrieking Shack) via the Whomping Willow is slipped past us and instantly buried as unfeasible. And how we are told it was planted the year Lupin came to Hogwarts.

The scene in the pub was full of interesting tidbits.

And we are told that a spy for Dumbledore tipped him off that Voldemort was after the Potters. This will play a part in later books.

Regarding the earlier discussion (in the first book), of what type of Fidelius Charm was used to hide the Potters, Flitwick's words here seem to contradict the possibility that the house was hidden:

You-Know-Who could search the village where Lily and James were staying for years and never find them, not even if he had his nose pressed against their sitting-room window! (p 153 in my edition)

This could of course be Flitwick slipping up or him not knowing how the Fidelius Charm was applied in this particular case, but it would seem to suggest that Rowling, at least at this point in her writing, thought of it as hiding the people rather than the house.

But Trelawny's predictions are still holding true. She says Lupin will not be "with us" for very long, and he leaves at the end of the book. Not what she meant, of course, but it follows a trend -- I like how much Rowling depends on double meanings of words or phrases in this book.

Hermione was of course right about Sirius sending the Firebolt. Just yet another instance of the value (wrong word, I know, but I still think it is a good one) of an incident changing entirely based on the misreading of Sirius as a character.

The revelation that Lupin was friends with James at school should of course cement him as a good guy, but because we have just been introduced to the story of Sirius the betrayer of friends, this is not as unequivocal as it might have been (to the first time reader, that is). And the reaction of Lupin to the name of Sirius after the Patronus Lesson is very ambiguous (which is one of the things I consider part of setting him up as a false red herring). I never doubted Lupin for a moment, of course
In part because all his other unexplained actions are accounted for by the information that he was James' friend.

I also like how the book keeps alternating between "all is well" and oh-no-oh-no-oh-no scenarios. The scene that prompted this is when Harry has mastered a (granted, flimsy) patronus, he gets the Firebolt, all is happy, they are reconciled with Hermione, and then WHAM, Scabbers has faked his own death again. Later on there is a similar moment as they leave the Shrieking Shack.