Snape and McGonagall come through so clearly in this story. Their complementary (and yet not quite matching) acerbic wits, insecurities, and fights to make their own way in the world.
I really appreciate the touches of "reality" in the story: Minerva's fight for witches' equality, Severus dealing with people being 'too nice' in the aftermath of Voldemort's first downfall. And, of course, the consequences of Albus Dumbledore's utterly insane expectations.