I'm not a big 粉丝 of Gryffindors. They seem to be so obvious and will state exactly what's on their mind. It doesn't keep one guessing. With Snape, there's an air of mystery around him; there's so much that's still NEVER stated in the books. There's so many directions to take his character: he hates and he loves, he is sly and he is brave.
I like the depth of him. As a person (not just a fictional character), I admire his intelligence, his strength, and his loyalty.
"Nice" is a trite word, regardless. Gryffindors generally think of themselves as nice, but they hold some of the deepest prejudices, even to each other at times. Of course, there are a few generally nice people (like Luna) and I like them.
Snape played his part so well that none of us could guess 或者 try to guess that his character was such in the end. He was an amazing person with a bravery greater than what we can imagine, and balanced his hatred and affection for Harry's well being in a planned efficient way.
He's a much 更多 complex character, and a lot 更多 interesting because of it. His nature is a mystery, and someone with a bit of mystery about them is always 更多 interesting than an open book personality. And being nice is highly overrated, in my opinion. Most of the other characters aren't that nice all the time either, but Snape's willingness to project his "unniceness" and wear it like a shield without caring what other people think about it appeals to me. He's not really that bad anyway. I've met people in real life who were much less likeable and did worse things than he did.
Being nice isn't everything - nice is not the same as being truly kind, which Snape is (though he doesn't 显示 it, making it even 更多 admirable). He protects from the shadows... like a ninja! XD Sorry, as a Narutard, I couldn't resist. He really loved Lily with all his 心 and he was a good person, but his life just got worse and worse. :( I can't blame him for not being overly friendly with Harry, the spitting image of that jerk James (apart from the eyes of course). Snape is also a very funny character, especially in the films when he's played 由 Alan Rickman. :D So yeah, characters don't have to be 'nice' to be likable.