I find myself frustrated in the same way as Pokemon - it would be nice to be able to explore the End without having to worry about being sniped. A perfect example of this happened the other day, we were pushing Sawine around, we went into the Nether. Slurth followed us and as soon as we ported in, he started shooting and killing me and matt. This was, if nothing else, inconsiderate, as we were having fun and definitely not expecting to start anything.
I recognise this was a bit of a unique situation, but by the same token, I would very much be in favour of voluntary PVP in all worlds, counterbalanced by a PVP timer, which would prevent someone flagging themselves off instantly while in combat. I don't mind PVP when I want to do it, but when I don't, it's the most frustrating thing to be killed, especially when you make an effort to get away, and the bastard goes at you anyway. </rant>