The holy trinity is well known. The DPS fight the mobs with damage. The Healer fights the damage from the mobs. And, the Tank fights the agro from the DPS and Healer. In World of Warcraft the formula is pretty well set for easy gameplay. Lord of the Rings gets a bit hazier with their use of hybrid classes, range tanking, and tank swapping, but for the most part it follows the doctrine of the holy trinity.
Guild Wars came very close to shirking the entire thing. Agro does not really exist like it does in World of Warcraft or Lord of the Rings Online. Each battle with PvE mobs is reminiscent of a PvP battle. Players have 8 bodies against the team of enemies to kill. Because PvE can feel so much like PvP (especially in comparison to the stark contrast of PvE/PvP in World of Warcraft and Lord of the Rings Online), ArenaNet moved away from the holy trinity to a more enlightened trinity: the three lines.