Slytherin Wins House Cup

I'm adding this to my collection of book-scene HP art, because it does go with the final chapter of the first book, when at first Slytherin was ahead. I, as a Slytherin was tickled green when we won, but I'm not really gloating; I think there are probably a lot of people who have multiple accounts to make sure the points go up.
Anyway, the picture! Very, very, very nice job. I like how your rendition is like a combination of your imagination, and movie-McGonagall-Snape.
The details I love: the way her hands are folded, as if she'd really like to make a fist and punch that smirk off Snape's face, but she's much too cool for that. She's not really frowning, but she has her lips pursed as she usually does when she's displeased. The candles, the goblets, and the window are very lovely.
Snape: hehehehe. I'm not a Snape fan, but can't help liking your portrayal. Those eyebrows! He looks the right age, too; some artworks portray him as looking quite old, and while he had a rough life that probably aged him faster than if he'd had an easy life, he's still a fairly young man in wizarding terms. The hair is also nice and lanky, and the hands, folded in smug pleasure are fantastic.
Lighting and shading: your lighting choices are fabulous, for it really looks as if candle light is reflected on their faces and clothing.
I don't think there's anything you could do to make this better, for extra details behind them or to the side would take away from the purpose of showing the two of them and their contrasting feelings.