Pranks VS Trolling: What is acceptable?

This could apply to pretty much any multiplayer game, but what brought this to mind was this article about trolling in Minecraft. It basically states that Mojang is going to be more clear in its terms about what is considered acceptable behavior in game.

Personally, trolling/griefing/bm/whatever you want to call it, is a big part of the reason I play so little multiplayer (in any game that has multiplayer function). I personally have no tolerance for people in the world who seem to derive joy from making other people's game experiences a nightmare. No matter what, I don't bm someone, even if they are acting horrible towards me. I might vent to friends afterwards, but I wouldn't reciprocate, much less actually instigate such behavior.

If I'm playing a game with a good friend, and they play a prank or something, something that is minor, reversible, etc, that doesn't bother me. I may or may not be amused, but it wouldn't bother me. By the same token, if that's all my experience is while playing that game, it ruins it for me. I have quit playing with friends before because they seemed to think that the funniest thing in the world was to play with a new player and prank/troll/whatever so much that the new player could not even get the hang of the game (thus reciprocation was an impossibility even if I had been someone inclined to).

But where is that line? It seems a very blurry line. What constitutes a good-natured prank? What is just being obnoxious? And I guess for me, why is it even funny when those on the receiving end are only angry or upset by the act? I guess I just don't get the humor.

What do you all think? If you find this type of thing funny, why?