I'm not too upset about AD itemization but you gotta do at least something about it, Riot
Most people on the boards are just crying about how AD itemization is so much better and the only armor they can get is Zhonya's. While I think it's being a bit blown out of proportion at how the itemization for APs isn't as good as ADs, there is something wrong when 
gives 45 armor but 
all together gives 40 armor pen.
The counter argument to this is "Zhonya's is one item and that's three so u dum" but most AP people get Zhonya's as a 3rd or 4th item, much later into the game than an assassin who has 2 of those 3 items already (one of which also doubles as a decent defensive item). So when you get a Zhonya's and your rabadon's, luden's, morello, sorc shoes, and idk void staff/rylai's or something like that, your Zhonya's is reduced to 1/3 of a cloth armor or, 5 armor. Let's say that they have an enemy Talon with 
.
The Talon has an effective ~51 MR after a 
and then it's further reduced by 15 from 
to 36.
Also, you gotta factor Talon and the mage in question both have their base armors and magic resists, but I'm too lazy for that.
The assassin gets 36 MR but the mage gets 5 armor in a normal build due to how easy it is for assassins to access armor pen. That's my only problem but I play ADCs a lot so my point is instantly invalid cause "assassins QQ".
In all seriousness though, Riot should do at least some tweaking to items to fix these issues, and I know, mages get upwards of 700 ap and will still do crap tons of damage through the Talon's magic resist but I mean, they'll still probably get one-shot by him before the Xerath/Vel/other random mage that everyone uses as a generic example can blow his load back at him.
P.S. civilized discussion pls P.P.S. prove stuff that I said wrong, I'll be happy not upset, that's why it's called a discussion!
,
where 50%+50 exactly doubles all physical damage (assuming targets have 50% damage reduction/100 armor).
Since running armor seals has always been a good idea and even 
gets 75 armor @18: you were looking at armor pen essentially matching critical strike chance and being the major secondary multiplier for AD Casters from seasons 3+
are all causing issues because they don't really hurt damage scaling and the DPS loss for persuing them isn't particularly high, where the boost to defensive stats is pretty substantial given how the game's balanced. You're getting a tremendous deal on defense.