All of you who think he is OP, please take time and read this.
Morde's main role is meant to be a tank. So, what is a tank?
In RPG-s, word tank determines a player with high survivability which draws attention of boss/enemy players while his teammates who lack survivability but have damage potential do their roles.
How does tank draw attention? Against AI controlled bosses he generates large amounts of 'aggro', which makes him a priority target. However, human players know this and know that priority is eliminating DPS characters. So, tank needs a way to make enemies to attack him.
Most tanks have some form of CC which helps them to draw attention:
Rammus, Shen - Taunt
Amumu - Stun, Snare
Garen - Silence
Maokai - Slow, Knockup, Snare
Taric - Stun
Mordekaiser doesn't have CC. So, he needs above-average damage for drawing attention. You know that he can and will punish you if you ignore him, so you have to take him out.
Imagine if he had low damage. He would be ignored in teamfights, and ignored tank is useless.
Now, let's take a look at his shield. It is not OP, as most tanks have a trait that boosts their survivability.
Sion, Shen - Damage Shield
Amumu - Physical damage reduction
Rammus - Armor/MR buff, damage return
Taric - Heal, Mana Return, Armor Aura
Singed - Health Boost, CC reduction
Garen - HP Regen, Slow Reduction, Damage reduction
Compared to these, Morde's shield is not that strong.
Of course, Morde can neglect tank role and go AP. There is a simple solution to this.
Just kill him. For that damage he has traded lots of survivability and will go down pretty fast.
Thank you for taking time to read this.