i'm not familiar with grey's anatomy so i can't really comment on their casting choices, but yeah things like race and gender get tricky. because of course you want to represent a diverse group of people, but you also can't write a couple characters and then randomly assign race to them, at least not in this context. a black woman who's a doctor will have had to work much harder to get there than a white man (or possibly even a black man), and the way she conducts herself will be informed by that.

and as for the stereotyping thing, it's such a delicate balance that i think it's hard not to fall into it. i know that i do it, too, and whenever i catch myself wanting to dismiss a character because she's "stereotypical" i force myself to stop and think about what it is that i don't like about her, and whether that's a result of the writing or of my own internalized sexism.