You receive additional talents, a bonus to the experience you gain as you kill things and additional rewards from max-level World Quests if you have War Mode on.
On modern World of Warcraft servers, you can toggle between PvP and PvE in any capitol city by turning War Mode on or off. You receive additional talents, a bonus to the experience you gain as you kill things and additional rewards from max-level World Quests if you have War Mode on.