Now, talking about open ended game design sounds all nice and simple, but it raises a variety of problems. The three problems as I see it are Grief Play, Economics, and NPCs.
Grief play is in some ways the easiest, and in some ways the most difficult of the problems. It is easy because the problem is obvious. Some people are assholes. The difficulty is deciding on how to manage such assholes. Now, the simplest solution is simply to deny them the opportunity. Unfortunately, I think that is also the worst solution. The reason I oppose such a solution is because, the essence of a good fantasy setting is conflict. Good versus Evil, Law Versus Chaos, Nihlism versus Creationism, these are ideas that are grounded in the conflict of ideas, and any good conflict needs a little violence to spice things up. Take that out of the game, and you get rid of true heroes and true villians. What is heroism without risk and sacrafice? What is villiany without treachery? In a world where conflict is regulated and directed by Realms or Instances or Combat Zones, there is no real risk, no real relevance to a battle, and ultimately the world never will change or evolve.
But if we allow players to cheat, steal and kill, what is to prevent the world from being overrun with grief players? In my opinion, the best way to deal with this is to give the players themselves the tools to limit grief play. This means give them the ability to track down criminals, the tools to imprison them, the means to succesfully corner and kill them.
In Ultima Online, the number one problem that made PKing so ubiquitous was the fact that it was so incredibly easy to do it, and get away without threat of retalliation. Two things made this so. First, and most dangerously, was the fact that players could essntially teleport at will half way across the world. This made cornering and killing PKs virtually impossible, since they relied exclusively on hit and run tactics. You could organize a party to hunt them, but even if you found them, they could instantly port away to safety. I think this was a huge flaw in the game (as was teleporting as a means of transportation in general), and is something I think should be avoided at all costs. If you implement equivalent D&D spells, I think they need to be severely restricted, otherwise PKing will run rampant.
One way to do this is requiring expensive material components. If someone wants to gate around, make them pay for it. If you want to run away, fine, but it will cost you a 5K gp gem. This will call into question the economics of PKing, and will require either a legitimate roleplaying desire, or serious risk.
The second way to do this is to create tools for tracking and cornering players. A set of spells and skills, such as tracking or various divination spells, should give the player the ability to locate and trap any player. Maybe they require "forensic evidence" like hair, which can act as a material component in a spell. Maybe the player simply needs enough information to describe the player when casting the spell. However it is done mechanically, I think it is important to create a system that essentially recognizes the need for law enforcement, and gives players the tools to do it.
The benefit of doing things this way is that you get to keep your villians! Nothing ruined UO more for me than hte loss of its villians. Eventually, the villianous side of the game became unplayable to roleplayers because the restrictions were simply too harsh for any normal roleplayer to deal with. This limited the roleplaying aspects of the game severely. The notorious thief and assassin Lerus couldn't realy steal or kill anymore. This just made the game less about roleplaying, and more about its unappealing hack and slash elements. If it is up to me, I would choose the former over the later any day, even if it means tolerating the occasional idiot.