Let me explain my case. I know there is a LOT of debate on how strong the Lich King truly was because, to be honest, I don't think we ever saw his full power. He toyed with us in our encounter with him in Icecrown Citadel before finally destroying us when he felt like it.
Regardless, I personally think Lei Shen, the Thunder King, and the Final boss (on normal mode) of The Throne of Thunder raid in 5.2 is as powerful as he.
The Lich King had Frostmourne, along with his powerful Death Knight, Necromancer, and natural ability, was an extremely powerful entity. He could raise the dead, control the very winds of Ice and Frost themselves, bend Death to his will, create pools of Darkness that would swallow his foes, along with insane physical strength and endurance, not to mention a massive amount of forces at his disposal.
Now, the Thunder King Lei Shen, had a Helm that would strike fear into his foes, along with a Spear that could strike like a Lightning Bold, and an Axe that when threw, would have such accuracy it would tear through his foes like soft tissue, and crash like Thunder. One can only imagine the devastation this man had. Not to mention he commanded an entire legion of loyal Mogu with their own massive capabilities in power, and an array of Artificial Constructs crafted from Titan Technology he stole or found.
Lei Shen also had an artifact called the Divine Bell as we know, which would channel the Power of the Sha, and from what we knew of him, he could perfectly master it, amplifying his power even further.
Now from what we've seen from his encounter abilities, Lei Shen can literally absorb electricity itself and bend it to his will, while using it to an insanely powerful degree of making massive spheres of Lightning, forming whips of crackling energy, and you get the picture.
Both have an insane amount of physical power, possess extreme mastery over their own specific elements, whether it be the Lich King's Frost/Death/Necromancy or the Thunder King's degree of Storm mastery, and each commanded their own armada of super powerful creatures, from Peons to beings like Kel Thuzad or Elegon (even if Elegon was simply a Titan Device, we can all see how powerful he was considering how much trouble he gave us "adventurers").
On a story basis, both were "resurrected" to some degree. The Lich King laid in wait in the Frozen Throne, unable to do anything except give telepathic aid and madness to further temper Arthas to his will, while although the Thunder King was slain, he was brought back to life, and now his Wrath, like the Lich King's, falls upon us. Both led their people with cruelty and absurd power, and had their forces lay waste to all enemies who opposed them.
The bottom line is that these near deity like figures actually have a lot alike.
I know, again, the Lich King's true power is debatable, and for one Lei Shen's abilities, or rather their full powerful, are still up in the air due to not seeing him in combat.
But there are a LOT of parallels between these two that point to not only story based linears, but also power level wise.
So do you guys think Lei Shen is equal to if not a bit above or below the Lich King from what we know and what I've drawn for you here?