Nerfing Lethality.
So There was somehing called "flat armor penetration" Where you could get 19 from runes and with mastries and items have 20+ at level 1 , 40+ at level 9 , nearly 70 at level 18 But Rito decided to nerf it early game and give it more impact late game. So now you get 24 lethality from runes instead of 19 armor pen. and with mastries and items you could get at level 18 nearly 90 But then they nerf it and now you get nearly 70 like armor penetration used to be but with less effect early game
The problem isn't the damage or armor reduction. In any game in the world you should never have a window to react to an assassin . You should just expect it by experience.
Focusing on one aspect of offense should sacrifice all other aspects. So if you are full damage u shouldn't have durability . If you are full tank you shouldn't have high damage etc..
But the problem with the new lethality was that the items give alot of other stats that shouldn't be given .
The items are very cheap . easy to buy as it consists of 
Also the movement speed stacks somehow so with a
and
you have nearly 500 ms
Other lethality items increase that ms also.
Not to mention the build path that contans
for even more ms damage and tankiness
for not getting nuked.
Also
Gives ward detection and true damage that can reach 250 late game.
more movement speed low cd
Spell shield on demand. and magic resist.
So you are assassin that 1 shots people . that's normal actually very very normal. But whats wrong is that you have alot of defense along with the 1 shot. very high mobility from items not to mention your spells. Spell shield ward detection true damage from an easy cheap build path .
That was the main problem of lethality not the damage . it's what's added to it .
Assassins should be a high risk high reward role but with that build path it's lower risk high reward with less chance to get punished for a mistake.