The rule is really simple: it makes people uncomfortable when you make generalizations about natural traits shared by the group they're in. Period. All groups (even groups people feel proud to be a part of), and all generalizations, even ones that sound positive or don't apply to the listener. I'm not going to list examples but if you're having a hard time thinking of them just imagine overhearing a conversation at a coffee shop about "those <something you are>, they're always <something you do or don't do>."