Why Lethality is Bad Game Design
EDIT: This is from the perspective of a player that loves all the champions that have experienced massive changes due to the lethality changes. Pantheon, Wu Kong, Talon, Renekton, J4, etc... always been a fan of them.
First off, lets talk about the purpose of lethality. It is meant to allow AD Caster champions like Zed, Talon, Pantheon, etc... an opportunity to scale into late game. Prior to the changes, these champions would build armor pen, hopefully snowball and win the game before the enemy team got tanky.
By design, it is much more cost efficient to build defensive as opposed to offensive. This would result in a sweetspot of around 15-20~ minutes where champions that scaled off of straight AD and armor pen would have the opportunity to control the game. However, also by design, defensive items stack multiplicatively whereas offensive items can scale exponentially. This allows carries that build attack speed, attack damage, crit chance, crit damage, on hit effects, armor pen, etc... to melt even the tankiest of tanks. So, your AD/AP Caster would essentially carry the early game by playing mid lane and successfully making roaming opportunities. Sure, they could carry a game if they snowball, but late game you need your true carries... the guys that are scaling exponentially, to win the game for you.
Why should a champion that builds only AP/AD + Penetration capable of scaling into late game and cutting through tanks like butter? Shouldn't that be something that someone who builds attack speed, attack damage, crit, etc..... all that be left to? If AP/AD casters are capable of those numbers, that lowers the importance of having a true late game carry. Maybe this will change in the tank rework? However, if this is fixed in the tank rework, what was the point of the lethality changes in the first place if assassins/casters are going to be back to where they were prior to the rework? Why are ADCs building full lethality even a thing?
Lets now talk about the difference in design between an AP Champion and an AD Champion. Its simple really, both of their abilities scale off of 2 stats: AP or AD, and penetration (lethality) The key difference? AD improves auto attack damage, which is a massive % of early game damage. This is why champions such as Talon/Pantheon/Renekton are such a threat early game: Late game they are burst champions, early game they have the burst and huge auto attack damage. That is why you see Talon/Renekton/Pantheon so damn dominant right now, you literally buy your cookie cutter build... stack a ton of AD/Lethality, and hardly anyone is capable of trading with you early... and they don't fall off nearly as hard as they used to prior to the lethality changes.
Guys, AD Casters SHOULD fall off once the enemy team buys defensive items, otherwise you get what we have now: A fucked up mess where Lethality rules supreme. Flip side of the same token, they SHOULD be dominant early, like they were prior to the changes.