I love how I knew right away I was looking at a portrait of Snape! You emphasized all the right features; the nose, his dour expression, his complexion and even the dark clothes he always wears. The hint of green in his face feels right too. The way you've swept your brush from his robes to blend with the background lets my eye 'finish' the rest of it, if that makes sense.

As for improvements, I'd extend his chin to balance off the nose because he's a little top heavy, though if you're going for a full blown caricature you can make the nose HUGE compared to the face and get away with it fine. I'd also like to see a background or some texture behind him, but it works without it so that's just personal opinion.

This is a rather nice Snape. The overall feel I get from it is that it's a work in progress and just needs a little more time spent with it. I feel it could be improved by having greater contrast in the more important areas, such as his eyes and brows. Currently everything is about the same contrast and there isn't really a strong area that draws the viewer's eye.

The proportions of his face seem a little off. His chin may be a bit too small and his face too large in proportion to the rest of his head. Right now it kind of feels to me like his features either need to be exaggerated more, or the overall structure made more realistic.

I think this could benefit from having some variation in the background. Perhaps the area around Snape could be lighter, or a lighter edge could be applied around him. I like the crosshatching at the bottom of the art.

I would like to see this posted larger too, I think the small size detracts from the presentation.