MechaPilot
Explorer
Outright change - including Hermione's looks. Emma Watson doesn't have buck teeth, nor does she actually have a big, unruly mane of hair. As far as I'm concerned, casting a black woman as Hermione is just as insignificant to the role as casting a non-bucktoothed girl with manageable hair.
Edit: Come to think of it, since both of those factors (her teeth and hair) came up repeatedly in the books, I'd say that her skin color is an even less significant change.
Having read the books after seeing the films, I would agree. I was quite surprised by the difference in how she was described versus how she was cast. I just chalked it up to Hollywood, who is notorious for doing thinks like casting a good-looking actress for a role where people refer to her as being very plain-looking.