This is one of my most-hated abilities in the entire game.
It has so many really strong uses, while having no cost and a very low cooldown.
Point-and-click instant cast means that there's no counterplay to it other than "don't be near Lee Sin" - which you don't have much choice about if you are a mêlée champion.
Massive knockback makes him almost impossible for any mêlée champion to duel - he can just kick them away if he's in trouble.
Massive damage and ratios means he can use it to kill squishy enemies easily.
Repositioning tool allows him to do Insec plays.
And those aren't even the intended use case of the ability, which is to kick a champion into the middle of the enemy team to damage and CC them all.
Now, on top of all that, it gains tank-shredding power?
(Edit: or possibly the power to shred others with their team's tank, tree description is a little ambiguous.)