What's the line between bullying and legitimate criticism?

Think of it as you might like moderators on Fluther to behave. How critical can you be of someone can you be before it becomes bullying (a personal attack). Try to give examples of things that are just this side of bullying and just over the line.

Coming the other way, what kinds of comments are barely bullying, but still count as bullying? And is there a difference between bullying and a personal attack?

I’m trying to get to the gray areas where you think your example is clearly ok, but you think others might consider it to be bullying. Or where you think it is clearly bullying, but you think others might think it is legitimate. Examples are crucial here. Please don’t feel you have to defend them. Just lay them out for discussion.