IMO they should just put a message window on the quests to unlock WAR and DRK jobs that says "WARNING ! The ability "Provoke" from the Gladiator class is mandatory for some late-game content" before you accept it. That way, new players who play these jobs have absolutely no excuse for not having it, and it would be their fault for having clicked "Ok" blindly without reading the message. They could also just put it as a condition for the level 30 job quest, just like meeting the required level. "Requires level 30 - Requires the ability "Provoke" (GLD) as a cross-class".
Ooooooooooor maybe just make a taunt ability for each tank. This seriously should be the case already, it's a very basic tank tool in any game with a trinity system.
