I find SWTOR's use of phasing to be fascinating.
The world changes. NPCs do different things and even die. But the areas where the changes occur are cordoned off from the rest of the world, and are explicitly marked off using red and green force fields. If you enter one of these areas, there's an explicit note on the UI, telling you who controls this reality. It's generally the first person in the group who enters the area.
To put this in WoW terms, imagine if all the changes to the throne room of Stormwind in WoW's history still existed. Bolvar and Lady Prestor start there. When Onyxia is revealed in the Great Masquerade, it phases to become just Bolvar. Then when Varian returns, the phase changes again. But the throne room would have an explicit entrance. When you enter, you get put in a specific phase, and you know who's phase you are in.
WoW uses what I call "seamless" phasing. The world changes, and you really cannot tell where the change starts, or who's reality you are seeing. Or more accurately, you are always seeing your own personal reality. Two people in the same group can be in the same area, but be out of phase with each other.
By having the explicit entrance to the phased area, SWTOR has its phasing be less seamless and more like instances. Which makes it seem more gamist and less world-like. But this system has the advantage of making things clearer for group play. And parts of the world still change in response to events.
The problem with phasing has always been, given a group of two players with different states, determining which player's reality should hold for the group. I have seen many algorithms and strategies proposed, and they all have some flaws.
SWTOR chooses to delegate the decision on which reality to use back to the players. It is a very interesting strategy. It is a lot easier to deal with, at the price of making the world less "world-like" and more "game-like".