Asheron\'s Call has an interesting PvP system. You touch an evil shrine and you become PvP. In order to go back to PvM mode, you must have NOT engaged in any kind of PvP (that means not even one swing or spell targeting a player, unless it\'s a buff, obviously) for 24 hours in-game (not game time, not 24 hours, but 24 RL hours of in-game PLAY), and then to touch a good shrine. This prevents you from becoming PvP, killing someone, then immediately going PvM to prevent reprisal.
For this game, I think that PvP shouldn\'t be possible if you\'re under a certain level (to prevent the noobs from being slaughtered and generally picked upon.) After that, there should be a level range that you can PvP with, say 5 levels or so in either direction (not sure how much of a power difference there are between levels yet, I have to get someone to download the client for me to play since I have dial-up). This makes it so you don\'t have the vets preying upon the lower level characters (a major problem with most PvP, in my opinion.) In all cases, PvP mode should be an option, if I don\'t want to have to deal with it, it shouldn\'t be forced on me (takes away from the realism which I don\'t like, but it\'s for the best since PvP can really ruin some people\'s enjoyment of the game. I remember in the old online NWN on AO-Hell many times someone asked for help in a battle, I\'d go in and help, and as soon as the monsters were dead the SOBs would turn around, gang up, and kill me; it really killed some of my enjoyment of the game.)