Southshore used to be Alliance and now it's Horde. Theramore is destroyed. The Barrens is split in two sections by a huge chasm. The Vale of Eternal Blossoms no longer has blossoms, eternal or otherwise. We've seen a lot of changes over the years we've been playing WoW, particularly during Cataclysm. But we as players didn't bring about these changes. They were written into the story and implemented in game.
But players can make changes via phasing. Many places in Wrath of the Lich King and beyond change according to what quests the player has completed. These are permanent alterations made by the player, but its individual for each player. Changing the environment for one player does not change it for all.
We, as players, can make changes to the world via world PvP locations. Halaa, for example, changes faction according to who battles there. But, unless you are on a PvP server, you have to flag PvP to do it and the changes are not even semi-permanent. They just last until the next occupation or battle.