I always thought Clerics only felt overpowered because of their DPS. Or burst damage, since all their attacks become fatigueless.
Since they take longer to kill than others, their DPS should also be lower. Giving DK bolt fatigue would be more fair than nerfing the damage, which could just make them unfun to fight with. Cleric ley is so low outside their shrine it's not a good idea to make their attacks require more power for the same damage. But giving them a slower attack rate would effectively stop them from doing burst damage comparable to fighter classes.