Aside from the zipper moment (whih I knew everyone would mention) what I like most is Buffy here.

She's long ago given up on being vulnerable, it doesn't seem to have any part in her life. But Angel is slowly drawing all that out of her and her reaction to it is perfect. She's frightened by it, angry that he's making her feel this way (and angry that it's a vampire), grateful that she still can feel this way and very confused by it. You show all of that and I love that Angel doesn't push her on it.