Pain as flavour
With the level 30 cap, many players will continue existing characters while others want to try something new. For new starts there is an effect many probably missed at the beta launch: Forsaken Paladins take damage when they cast Holy Light for the first time.
Forsaken Paladins are a brand new race and class combination in World of Warcraft: Forever. One of the first class quests is called Rediscovering the Light and teaches Holy Light.
A bug stood in the way
In the quest you must heal 5 Injured Deathguards with Holy Light. The quest text explains that using the Light on yourself or others may cause considerable pain, and every cast deals some damage to the Paladin. It will not kill you unless you are already close to death, but it adds flavour to the quest.
When the beta launched, players did not take this damage. Josh Greenfield (Aggrend) explained that the quest was always meant to hurt. With the fix now in place, the original vision can be experienced.
Original report: Wowhead