Harry. He is getting used to having Ginny around. Awww.
I also like the moment when he chooses Neville and Luna explicitly, not as a default "they are the ones that are around" thing.

Romilda Vane. Ick.

Malfoy and Pansy Parkinson. Ooooh. What is going on there? I had not noticed that before.

I also felt like going "hah" when Malfoy found out he had been dropped in favour of Neville. Mostly because Malfoy is not aware that Neville has made such great strides in the past year (which, granted, is not why Slughorn invited him, but it must add to Malfoy's chagrin).

Snape: his comment about Tonks' new patronus looking weak -- no surprise considering his history with the marauders. It works on both Sirius and Lupin, although the comment is probably more hurtful in Harry's interpretation of it as Sirius.

I also liked Hermione's observation that Snape's comments about the dark arts sounded like what Harry says. The two of them do have things in common, and I like it when that is made clear.

Also, did anyone else notice that Lily was good at potions? I cannot help connecting that to Snape.

Lavender Brown fins Ron funny. She has not been that happy about him earlier.

I love, love, love McGonagall. The way she supports Neville while talking about how "Augusta" failed her Charms OWL is wonderful.

Hermione and Slughorn's interaction is lovely. It shows both in a good light. I also liked how anything that makes Harry better at potions than her must be Evil™.