How much is too much armor penetration?

What the huh? That makes no sense whatsoever. Dunno why I wrote that.

Ashe is the only one I can think of off the bat. But she's not really a caster, anyway.

I agree that Flat Pen is a weird stat in that people would generally think, "Oh Armor Pen, that must be good if they buy armor" even though the opposite is true.

You mean in the sense that %ArPen gives more ArPen per gold than flat ArPen when the target has a sufficient quantity of armor, right?

Since the damage reduction formula is non-linear, I used to think that buying Sorc Shoes against a 30 MR target was much more valuable than buying them against a 200 MR target, because you'd see a much larger change in percent damage reduction in the former scenario. But then I sat down and did the math, and it seems that each point of armor or MR increases your effective HP by 1%, regardless of how much you have. (The reason why you want to balance Armor/MR/Health is not that Armor/MR gets "objectively" worse, but that each point makes Health more valuable, creating optimization points along the way.)

But that's such a weird conclusion that I keep doubting it, even after running the numbers.

Well, tanks are still effective and strong. Maokai was the #1 top laner for the entire world championship.

Being literally unkillable is not a trait that we think is acceptable in League of Legends, so Last Whisper and Void Staff exist.

I agree that Flat Pen is a weird stat in that people would generally think, "Oh Armor Pen, that must be good if they buy armor" even though the opposite is true. But, the idea of counter-building back and forth is I think a pretty big key of the item system.

Maokai has a % damage reduction ability that is not affected by armor or magic penetration.

This fact is highly relevant.

The core problem is that the game is designed, basically, on the assumption that equal GP of defensive stats should be defeated by equal GP of offensive stats. And then added to that, the offensive characters get more gold (because kills are worth more than assists). The result is that in early and mid game, tanks can work great, because they are not significantly behind in gold and will commonly even be ahead.

However, as lategame approaches, the gold will either even out or cause the offensive character to be ahead. Further, the offensive items have more and better multiplicative scaling (it used to be much more even before the Randuin's Omen attack speed reduction nerf). At six items, the offensive character will shred the tank without any major difficulty. That's the problem.

No one wants a tank to be able to ignore an ADC like they're not there (unless the tank is super far ahead). We want a six item tank vs a six item ADC to play out the same way a three-item battle would; the tank is resistant to damage but can die.

Hello phreak first time replying to you. Anyway what he meant is that tank stats are balanced around people being able to buy that item which should essentially be an anti tank item. If you converted it to an anti tank item only (instead of always build this item) then it would mean that tanks could be just as strong against non tank shredders as they are now (or maybe even more so) but less tanky against champions who actually invest into items specifically for shredding tanks. Incidentally this might also fulfill OP's dream of finally being like Shredder when he plays Zed. Hope this clears things up.

My point is in no way that Last Whisper/Void Staff should not exist. In fact, I'd be very sad if they didn't.

What I'm getting at is that currently, both items are efficient versus EVERYONE, rather than being a choice you buy to defeat tanky players.

Stacking AD or Flat Pen should be the answer to low-defense enemies, whereas buying VS/LW should be the answer to enemies stacking defenses, but that is not the case in League. There's no reason NOT to pick up LW/VS just as a default part of any build, because they're both very gold efficient and scale well. That's what I'm getting at.

There's no real meaningful choice (IMO mostly from lack of options) between flat pen and %pen, because %pen items are already efficient vs the same people that flat pen items are supposed to be more efficient against. Is that more clear?

I hate unkillable champions with a passion, I'm just saying the relationship between flat pen/%pen/damage is currently fairly flawed at it's core, resulting in some problems with itemization that don't make a whole lot of sense.