The most notable change in this area from Diablo I to its predecessor was the change from the persistent friendly fire of Diablo I, where any projectile could hurt your comrades or simply other players in the game, to the system of Diablo II, which required players to declare hostility against other players. With this came even further changes as the game grew older, including patching of some side affects that were not fully handled in the initial release of Diablo II. This included things like magical projectiles being cast before a player clicks the hostility button and then instantly killing other players unknowingly. Other restrictions included restricted access to Town Portals and Way Points.
Other issues which were still never addressed are some continuing aspects of griefing, which is often exploited by more powerful characters on the weak, the spamming of health, mana, and rejuvenation potions during duels, and the heavily controversial gift of Teleportation, an original skill of the Sorceress, which was allowed to be used with any character until the introduction of the Enigma runeword, which was introduced in patch 1.10.
What needs to be better with Diablo III's PvP system? What was bad about it in previous games? What new mechanics should be introduced to it to make it a more viable time-consumer in the end-game?