Robbie Coltrane as Hagrid is a stroke of pure genius. If you've ever seen him in "Cracker", you will notice this role shows his versatility, since the character is diametrically opposed to that role. I also adore Maggie Smith as McGonnagal, she is so intently the teacher, that I want her to be my teacher. Of course, the irrasable whatshisname that everyone is drooling over is good too. He just has to present, and suddenly he can't be ignored. It's the way he stands, girls, you just know he's got it. His character is so obtuse, it's difficult to pin him down as a really intimidating good guy, or a dastardly bad guy. He takes his time. That's his charm.
I can't say as I recall my impression of Snape when I read the 1st book (I read first, saw movie later). I think he was meant to be imposing and intimidating, since what the school is up to is quite dangerous when you think about it. (All that power being taught to children.) I didn't have time to read the rest of the books, except here and there, so most of my HP storyline experience has been the film. I can't say anymore than that. JK is extremely imaginative.