The Mountain Troll scene is one of the most important in the book, because without it, Harry and Ron would not have become friends with Hermione. It’s interesting how this kind of life-or-death situation often brings characters closer together in Harry Potter. You could say the same thing about the battle at the Ministry in Order of the Phoenix. At the beginning of that year, Harry was embarrassed to be sitting with Luna, Neville, and Ginny on the Hogwarts Express; by the next year, he’s proud to say that they’re his friends and turns down an invitation to sit with Romilda Vane and her crowd instead.
This section is also when Rowling really starts trying to make us suspicious of Snape. Harry overhears him telling Filch about the three-headed dog, and in the quidditch game, it certainly appears as if he’s putting a curse on Harry’s broom. Of course, later, we’ll learn that he was trying to get to the stone before Quirrell, much like Harry and his friends do, and that he was actually performing a countercurse to try to save Harry while Quirrell was the one trying to curse him. We also get another big clue about Professor Quirrell, though: when Hermione sets Snape’s robe on fire, he’s knocked over. The first-time reader has no reason to even notice this, but it’s there, nevertheless.
The third chapter I read today was The Mirror of Erised, and I’d almost forgotten how different it is from the movie version. Superficially, it’s pretty much the same: it shows Harry his family and Ron himself, accomplishing more than any of his brothers. But in the movie, we don’t see Harry becoming so obsessed with it, refusing to play chess or visit Hagrid, entirely losing his interest in Nicholas Flamel and whatever Fluffy is guarding … it reminds me a bit of the Island of the Lotus Eaters from The Odyssey, in that Harry thinks it’s making him happy, but it’s an illusion that’s drawing him away from everything that used to be important to him.