If stealth in PvP is anything like I've seen in other games, when you're far enough away you simply won't see the stealthing player... but if they get close enough you'll see their translucent outline/image. I ~seem~ to remember something about how in PvP you can see a stealthed player's shadow on the ground? So to be really efficient a stealthed player would need to stay in shadows / darkness to hide her shadow? Maybe that was just something someone ~wanted~ to have, I don't remember.
RE: the OP, though, one thing that frustrated me --- but that I also loved --- about Thief style missions is when I was under orders NOT to kill anyone. I think Oblivion did this (although it MAY have been a thieving mod I added later)... and other sneaky type games basically forbid you to kill anyone, either because that will alert them that someone was there and you are not supposed to be detected AT ALL or because "we're thieves, not murderers"...
Of course, in an MMO where you can pick up a mission and then go off and do something ELSE, you would have to adjust this restriction. It would work best in an instance, of course --- once you enter the "grounds" of your target's mansion, for example, you cannot kill anyone or you fail the mission (or get a lesser reward or something).
Perhaps it could even be an "undocumented" type thing --- if you HAPPEN to sneak through a steal-this-for-me mission without fighting anyone, using pure stealth, you may unlock some kind of Ghost achievement or perhaps unique dialog from the mission-giver: "They didn't even know you were there! You're quite the thief!"
It probably goes without saying, but on the UI front, I hope they make it clear when taking an object would be considered stealing --- in the TES games, the interaction icon would be a red hand, for example, to let you know you were about to be stealing something. Likewise, I hope stolen goods are flagged as such in inventory, so people don't wonder why they cannot sell their stolen goods to Honest Joe's Supply Shack and instead have to find a Fence to sell those stolen goods....