I adore Scratch and the Owlbear cub. I haven't enjoyed animals in a game as much since DA:O.
I love the little surprise comments every now and then when you look at a book or pass a landmark with certain companions in the group.
I LOVE the fact that there are little details like Astarion actually taking damage from flowing water! He even refuses to cross or tries to find another way around which has gotten me into some really interesting situations a few times!
Also the little abandoned picnics dotted around in random spots, notes that make you feel ABSOLUTELY awful if you take them from a freshly murdered corpse.
The gods damned windmill if you hit the wrong button

Larian included these little rewards for exploration and taking notice in DOS2 as well, and I love it.