How is donning the Do-Not-Disturb tag rude?

As for the DK's...Yes, they can be quite annoying. Mainly because they're everywhere, not that I've had much trouble with them in the starting zone.
Towards dueling...what's the point, really? I had this level 80 tell me to "duel him" because I wouldn't join his guild on my four extra characters.

Really, what's the frigging point?