Excuse my newbishness;
In my own opinion,
Open PvP encourages teamwork and evokes a sense of danger. For instance; crafters will obviously be targets because of the relationship between the two; the pvper produces nothing but requires something, and has developed a means to obtain it, while the crafter produces things but (usually) lacks the means to properly defend it. Because of this, warriors aligned to an organization will find they have an important job; protect the crafters.
Town guard KoS list (Karma system featured in the game could also help accomplish town security should a PvPer's karma drop too low, causing NPCs to attack.) and/or other mechanicisms would of course be welcome in aiding the defense against PvPers, and ways to protect items, such as locked containers and/or 'bound' important items, so to speak, which could last for one or two deaths before requiring a new 'bind'.
Restricting a sandbox game reduces player enjoyment, imo, and while dying may be frustrating, a game without death is hard to pull off, especially in such a player oriented game. (ATITD is an obvious exception.) Even restricted PK/only animal deaths could be less preferable compare to open PvP.
on another note;
Automatic game punishment of pvpers would also make the mechanic a much less enjoyable one, whereas player-imposed punishments assisted by game mechanics can be quite satisfying and add an element of danger to the pvper as well.
Hope I could at least make one point there.