Snape/Moody is my favorite m/m pairing by far, and this story is one of the best I've read. I love the overall premise; it reminds me of "Waiting for Godot" and "No Exit" and all those clever tales where everything beyond the here and now is a tantalizing mystery. I love the way you place subtle clues throughout and yet still leave things wonderfully ambiguous.
Both Snape and Moody are beautifully characterized and complex; your prose is sharp and tight; the dialogue crackles; the ending is just what it should be. What an excellent job.