Edited: 12/31/2013 3:15:39 AM PermalinkThe answer is layers, shards, and phasing. It allows players to exist in the same "physical" space without being visible to each other. From the player's perspective no one outside of their fireteam exists, until they enter a public bubble and/or crossroads space.
So there you are at the Tower and you're in your third person mode walking around, Gaurdians are sitting on the steps or running to and fro. You notice a master, one of those white clad Gandalf looking NPCs and gathering around him are two or more players you walk over and he begins laying down a quest for Mars and the liberation of an exotic weapon. You get the mission and those other players run off to get in their ships for a cutscene (yaaay...not really) Anyways moving on you cut scene into flying over Mars(arggh how do you say manual flight isn't in? It's just so anti inner kid) and you teleport to the ground and there you are in front of a building covered in red sand. You gotta go in and head to the lower levels. Now standing there is seven other players looking at the same window. If two of those players are on the same mission how do you know they are potential allies if they are not talking? Is there a way you can coordinate with them like in BF3 when you join a squad and they are highlighted in green? Not everyone is going into that building for the same reason, so how do differing missions within the same local affect each other? I mean that would be hilarious if you had a stealth mission and the other players had to go in guns blazing because they are on a different chapter in the campaign. Everyone is stepping on each other's toes and to me whata story that would be. Anyways just musing over the image of a bunch of players on Mars trying to figure out how to tackle a fortress thick with enemies buried under the red sands.