The difficulty with armor penetration (or ArP) at launch was fairly simple. The stat itself was extremely expensive for any significant benefit before patch 3.1, so that the only real item with significant ArP anyone made much effort to acquire was Grim Toll. Furthermore, with Titan's Grip in the state it was in at that time (with no penalty), warrior damage (especially fury, which had access to TG) simply overcame armor mitigation with brute force.
But in Patch 3.1, three things happened that changed the game for warrior DPS and armor penetration. First off, the 10 percent damage penalty was applied to Titan's Grip. Due to the way rage is generated via white hits (that is, attacks that are not special, rage costing attacks), the loss of 10 percent white damage actually cost fury warriors at that time closer to 25 percent of their total damage. The second was the increase in the effectiveness of armor penetration, getting 25 percent more benefit from the statistic. Third, and probably the most important, was the sudden influx of armor penetration on gear available via emblems or in Ulduar itself.
Armor penetration became extremely important for pure physical DPS at this point, be it hunters, warriors, druids, rogues or what have you. Since warriors are one of the most physical of the physical DPS classes, it quickly rose to prominence for us. However, the stat has never been what you might call intuitive. Before long, the developers realized that without a cap on it, theoretically, you could reduce armor with ArP to the point that targets effectively had negative armor and took even more damage from an attack than it actually hit for. Furthermore, the buff to armor penetration in patch 3.1 turned out to be, shall we say, extremely generous. It was possible for a dedicated player to stack ArP to at least the soft cap (50 percent passive armor penetration plus a proc trinket like Grim Toll or Mjolnir Runestone) -- since the hard cap was 1,232 armor penetration rating before patch 3.2.2, it wasn't hard at all for someone in Ulduar gear to stack up 616 or so ArP, and then GT or the Runestone would reduce armor beyond the cap when they procced.
Patch 3.2.2 reduced armor penetration rating to the current level, where 13.99 rating was equivalent to 1 percent armor reduction. Combined with the 100 percent cap on armor penetration effects, this meant that armor penetration now had a relatively high but attainable hard cap where it would do absolutely no good at all, and that number was 1,400 armor penetration rating. (Before patch 3.2.2, it was 1,233 ArP rating.) From that moment in the midst of clearing through Trial of the Crusader to the opening of Icecrown Citadel and to the present day, for a DPS warrior (especially fury), the goal has been fairly simple. Gem and gear to hit 1,400 ArP, then stop. As soon as you hit that magical number, gem for strength. While you can't actually reduce armor to nothing (according to Ghostcrawler's explanation of the new mechanic, which was implemented to keep armor pen from returning to its BC days as the ultimate cloth-killer stat) as soon as you hit 1,400 rating, you have reduced armor as much as you possibly can, and the cap will prevent you from reducing it any further.
While armor penetration is good for any class that does physical damage, the reason it was so potent for warriors is again our rage generation system. Armor penetration allows all physical DPS to hit harder, bypassing more armor with their attacks. But not only are warriors one of the classes who do the most purely physical damage, but doing that purely physical damage with a white hit will also generate more rage and thus give us more resources to spend. Marksman hunters don't get mana back for stacking ArP. Feral druids don't get quicker energy return for stacking it. But a fury warrior with a ton of ArP can fill his rage bar so trivially that there is absolutely nothing preventing her or him from simply using every ability while keeping Heroic Strike queued constantly.
For illustration, go watch a fury warrior on a target dummy some time. Specifically, watch his rage bar. Just standing there, whacking at a dummy, an ICC-geared fury warrior will most likely have between 85 to 100 percent armor penetration. Each white hit will non-critically hit for close to 3,000, meaning that it will take one swing of those great big weapons to almost completely fill up his or her rage bar. This allows the warrior to keep HS queued up, which has allowed fury warriors to severely undervalue hit rating this expansion, as their miss chance while using HS is only 8 percent instead of the close to 24 percent it would be simply dual wielding -- keeping HS queued keeps that miss rate even for white hits. This allows them to gear for purely damaging stats like, you guessed it, ArP. Effectively, ArP counts double for us. It's much the same phenomenon as when the reduction of damage via Titan's Grip's penalty in 3.1 cost us more than the stated amount lost: When you lose damage, you lose rage. When you lose rage, you lose more damage. When you gain damage, you gain rage, which you can immediately turn into more damage. White hits go up, so too does everything else.
It's not just PvE that finds warriors using a lot of ArP. PvP warriors have ended up gearing as much as they can with raid weapons and trinkets, be they arms, fury (LOL) or even protection. You'll often see protection warriors in a mix of PvP set and PvE trinkets, rings and ArP weapons to bypass as much armor as they can in an attempt to not only generate more rage (having rage ready to go in the burst-heavy world of PvP is very important) but to again blow through cloth wearers' armor.
ArP has gone from being a junk stat that no one really wanted to one of the most powerful stats in the warrior arsenal. Its loss isn't a bad thing for the game overall, I don't believe, but it will require a great deal of adjustment for warriors used to relying on it to reach parity with other classes that can just ignore armor for some or all of their attacks. We've yet to fully see how the new, rage-normalized, always-affected-by-armor warrior really shakes out.
Check out more strategies, tips and leveling guides for warriors in Matthew Rossi's weekly class column, The Care and Feeding of Warriors.