It would be nice if there were a stated set of default loot rules created by those running the server. Groups could still agree before running the dungeon to follow something different but at least it would create a baseline expectation so players know before hand what to expect from each other.
I know there are many philosophies about how to handle loot so it's impossible to come up with something which would solve ALL problems and address EVERY issue but at least it would create some accountability about working as a team. It would also provide some recourse to players who are performing their role in good faith expecting they'll be given the same chance for gear as the other group members.