From WoWWiki
Leather armor is generally worn by rogues and druids, throughout their full WoW "career", also by hunters and shamans while they are below level 40. Leather armor provides more protection than cloth armor, but less than mail or plate armor. At level 40 both hunters and shamans are able to wear mail armor. Warriors and paladins can wear leather, but usually choose not to, due to its lower defense value than mail and plate. However, a Warrior or Paladin specced for DPS or healing may choose to use Leather Armor if the stats are an improvement in damage or healing; as a tanking class will be taking most of the damage in a group, the high mitigation provided by plate armor may not be necessary for a DPS or healer.
See also
Leather armor is generally worn by rogues and druids, throughout their full WoW "career", also by hunters and shamans while they are below level 40. Leather armor provides more protection than cloth armor, but less than mail or plate armor. At level 40 both hunters and shamans are able to wear mail armor. Warriors and paladins can wear leather, but usually choose not to, due to its lower defense value than mail and plate. However, a Warrior or Paladin specced for DPS or healing may choose to use Leather Armor if the stats are an improvement in damage or healing; as a tanking class will be taking most of the damage in a group, the high mitigation provided by plate armor may not be necessary for a DPS or healer.
