There are abilities like Sunder Armor and Expose Armor that lower armor temporarily, of course, and the rogue talent Serrated Blades. Thing is, I'm not too sure who I'm threatening, you or me. For that to be true, you need to hit the break even points against armor. Of course you can get 2 AP with the same item budget that 1 ArP would cost. The effective armor on the target is I'm just using these numbers as examples.
Debuffs and personal Armor Penetration.
The Care and Feeding of Warriors: Armor Penetration
My first conscious exposure to the mechanic was the epic weapon Bonereaver's Edgewhich dropped off of Ragnaros. A level 80 warrior creature has armor. This also means, that it scales with anything that increases your DPS, including talents that increase your Str or AP. In short, armor penetration is really only countered by more armor. All times are GMT. Thing is, I'm not too sure who I'm threatening, you or me.