Consider dealings with agents that have morals that conflict with your own. Obviously, major value conflicts preclude co-existence. Let's assume it is a minor conflict - Bob believes eating cow milk and beef at the same meal is immoral.
It is possible to develop instrumental or terminal values to resolve how much you tolerate Bob's different value - without reference to any meta-ethical theory. But I think that meta-ethical considerations play a large role in how tolerance of value conflict is resolved - for some people, at least.
Obviously, major value conflicts preclude co-existence.
Not obvious. (How does this "preclusion" work? Is it the best decision available to both agents?)
Do you believe in an objective morality capable of being scientifically investigated (a la Sam Harris *or others*), or are you a moral nihilist/relativist? There seems to be some division on this point. I would have thought Less Wrong to be well in the former camp.
Edit: There seems to be some confusion - when I say "an objective morality capable of being scientifically investigated (a la Sam Harris *or others*)" - I do NOT mean something like a "one true, universal, metaphysical morality for all mind-designs" like the Socratic/Platonic Form of Good or any such nonsense. I just mean something in reality that's mind-independent - in the sense that it is hard-wired, e.g. by evolution, and thus independent/prior to any later knowledge or cognitive content - and thus can be investigated scientifically. It is a definite "is" from which we can make true "ought" statements relative to that "is". See drethelin's comment and my analysis of Clippy.