I think the thief should have to grab the player for a certain amount of time in order to steal something. Also, the items they steal should depend on how well they steal, which could be based off of their steal/attempted steal ratio. (how many times they have tried to steal and how many times they succeeded)
Plus there would need to be protection from thieves, as I would probably quit playing if some guy comes up and steals my one of a kind war axe.
I would imagine levels could play a factor here... Like if the amount of time it takes to steal something is based on the level the thief is at compared to the person he is stealing from's level.
So if the thief is level 3 and he tries to steal from a level 9 guy, it would take a considerably long amount of time, holding on to that level 9 player, before the thief could manage to take anything. Then the "success" could be determined by if they retrieved the item they have stolen.
I would hate for some guy to just come up, touch my back, and take some valuable item. I mean sure you could hit him and make him drop it, but whats stopping him or someone else from picking it up before you can get to it? I mean if two guys work together to steal stuff, one guy could just wait until the other guy gets hit and then run up and get the item.