Well, you hit so many of my kinks with this fic--student/teacher, May/December, fluid sexuality, first-times (or close to), plus Dumbledore/McGonagall--I probably would have loved it even if it hadn't been gloriously realized.
The buildup of the relationship between Albus and Minerva is fabulous; it's easy to see what these two people might see in one another despite the vast age difference. I love your Albus--a genius with a blocked head about people--and the observations about the effect he has on those around him is spot-on.
I also love the way you hint at the less attractive elements of his personality--the "ends and means" problem.
Horace is also a gem; he's wise and kind, yet recognizably the voluptuary and opportunist we meet in canon.