I agree that there should be some sort of cap that forces people to do class quests to learn their jobs and learn new abilities. Maybe there should be a level cap every 5 levels which will only be removed once that specific class quest has been completed (e.g. you can't go past level 19 until you complete the level 15 quest).

But also the class quests themselves need looking at. The Gladiator class quest (particularly the one at level 10) at least attempts to teach you your role in battle (in this case, using Flash to draw hate away from your NPC ally), but such quests are few and far between. For example, for the marauder quests, actually doing your job by drawing all your foes' enmity usually results in a fail because of the hapless NPC healer you have with you. In pretty much all the conjurer quests the focus is always dealing damage and never healing your allies. In fact you could easily complete those quests without throwing a single cure spell at anyone other than yourself.