Hah! It's always amusing to me--and a bit gratifying--when Cedric rises once more from the dead.
Yeah! This Raise cost me 5000gp in diamonds, it BETTER be gratifying somebody!
Even with mature, skilled role players, I still believe D&D is built around the model of the DM as referee. How tightly or loosely they should hold the reins depends very much on the group, but letting the reins go entirely shouldn't be a valid option, else why have a DM at all? And if you don't have a DM, well...if you're having fun it's not for me to judge you, but whatever you're playing isn't actually D&D at that point.
I believe that all drama is based on conflict and tension, if you can't come into conflict with the world around you, there's no drama, and if you can't potentially LOSE, there's no tension, and thus no valid conflict. That's why I bet most of us stopped playing video games with a God Mode cheat on at about the age of 10.
If you plan to make moral philosophy a part of your game, then it also becomes a part of your drama, which means that to remain fun, it must have the potential for conflict and tension, which means the potential MUST exist for the players to be wrong or just screw up, otherwise they will quickly realize that they can basically never break their code and lose class abilities unless they decide to have their character fall as part of the story they want to tell. And while a mature group of players won't intentionally abuse this, it's still a lot less fun because the tension, and thus the drama, involved in having to balance the code with the demands of combat and trying to succeed in your character's endeavors, is lost.
I believe it's a false choice to say morality is either player defined, purely procedural, or "guess the DM's opinion". I believe that a skilled DM, one who is willing to take the time to discuss a little philosophy with their players beforehand and to use their world and their storytelling role to convey to players how they intend the universe to work, as well as give the players some leeway to have a say in the matter without just saying "do whatever you want", can create a world that makes enough sense and has strong enough recognizable philosophical themes that players can interpret it if they try and figure out what is expected of their characters, without the answers always being immediately obvious.