Armor pen vs Armor shred
I see a bunch of posts talking about armor pen and how it makes playing tanks suck. That is a true statement, and while I do think armor pen should be nerfed (lightly), I think the biggest issue is armor SHRED. The difference can seem trivial, but applying % armor shreds for the whole team to enjoy can be extremely powerful, especially when multiple champions on the team applying that % shred can benefit from it. I think most instances of armor shred in the game should be changed to pen and nerfed lightly, but the biggest issue is the black cleaver. If only the champion with the black cleaver benefited from the % armor pen it would be a huge nerf to the item, but it would also have fixed a ton of the issues we've had since the black cleaver was changed. It warped top lane, which warped resists, which warped armor pen and crit, which warped damage output as a whole.
Personal opinion: I have always felt % armor pen is a necessary evil but that it should be attached to limitations not buy this one item and the entire enemy team loses 30% of their armor for the rest of the game.