I believe that more can be done to incentivize Paladins/Warriors to use Plate, Hunters/Shaman to use Mail, and Druids/Rogues to use Leather. Because armor itself doesn't play a huge role for several classes, particularly in a dungeon setting (a Paladin healer, a ranged Hunter, an Elemental Shaman), it oftentime happens that players from these classes want to need on gear from lesser armor types. I don't think this is great because cloth wearers can't do the same in reverse.
The best fix to this problem would be to more heavily incorporate armor amount into talents. The Octo devs did this with hunter pets, tying pet armor to hunter armor. There should be heavier use of this principle across talent trees and classes.
In a Diablo 2 mod I'm making, I'm incorporating this principle into the skill trees. For example, heavy armor gives Barbarians certain types of bonuses, medium armor other types of bonuses, and light armor no bonus.