Minerva/Severus is my favorite het pairing, and you've captured most of the reasons I love them: cleverness, banter, UST, intellectual equality, their Slytherin and Gryffindor essences (with all their strengths and weaknesses; their inevitably differing -- and similar -- perceptions), the way they test each other. The only thing missing is the hot sex, but you've left us some lovely blanks to fill.
Of course, it's not just the presence of these elements that make this story so good: it's the way you handle them. There's your excellent writing, your pacing, your sense of atmosphere, your skill at characterization, your clever backstories and inventive magic.
And the way he leaned against the fireplace was the height of exquisite, elegant, insufferable arrogance. Yes, I can definitely see this Severus.
The parents' letters and Minerva's responses -- hilarious and so IC.
In its place was an expression that could stop an army of Weasley Twins. Yes!
even though Severus glared in a way that could Petrify the Giant Squid. And now it's Minerva's (perfectly-phrased) turn
this whole friendship thing – dammit, it was uncharted territory. Give him a quiet stroll in the Forbidden Forest anytime. You knew where you were with Unicorns and Thestrals. Another IC moment.