I, too, like D/s fic where Harry's the sub. And what you said about them both being vulnerable--that grabbed me. We don't often associate Snape with that characteristic, but I think you're right. For Snape, it's his fear of it that's remarkable, while Harry, well, his vulnerability is is part of his attractive appeal.
I've enjoyed getting to know you over the past months, and being part of Team Dragon with you. ROAR!