For me, it means sort of sarcasm that is very difficult to retaliate against. Difficult to describe but i know when im on the recieving and of it, and i think i can be pretty passive aggressive myself sometimes, that is a bad thing.
I know what you mean about the nervous laugh, but PA people do this and try and make it sound like a nervous laugh, the difference is what has been said BEFORE the nervous laugh. "Sorry i am late, i just got stuck in traffic, it was an absolute nightmare, but hey, you know how it is" nervous laugh thats OK.
"Oh, don't worry about being late, get stuck in traffic did you?" half laugh = passive aggresive. OR at least i think.
Gunznroses - i don't think you are being very fair, i don't think otherpeoples was being insensitive at all, just describing her friends laugh, her friend may have tourettes, or she may just get nervous. Most people associate TS with swearing, but i think most people also realise that it not always the case.