Well, the fic is amazing, for starters. And yes, the ideas are great, and the plot is wonderful and subtle and the details of Draco's reactions are brilliant.
But I honestly believe this is the best-paced fic I've read in AGES. The flow is absolutely perfect, and not only are there no snags at all, it moves through at a pace that feels so natural.
It's also following the #1 rule of short stories: Everything is important, nothing is unnecessary. You've got everything you need in there. Every word and sentence is absolutely essential to the story.
And on a more personal note, I love the idea of balance in Draco's life. I also like the fact that he never confronts Rita about his aunt, or asks her about her past. It's as if they're both just going on from where they are, and that's it.
You've really outdone yourself this time. Really. I honestly think this is some of the best writing I've ever read from you.