Dec 29th 2008 12:26PM After 80 comments I doubt anyone is really still reading these, but I really want to say I disagree that armor penetration is crap. I liked what it was at 70 and at 80 its different but still seems cool and i think it definitely can be better than crit.
15.38 armor pen rating = 1% armor ignore
45.91 crit rating = 1% crit
so in same amounts of rating, any X% crit = about 3X% armor ignore
the sunder debuffed boss has 9075 armor, with 10% armor ignore it has 8167.5 armor
9075 armor = 37.33% physical damage reduction, so your physical damage is cut to 62.67%
8167.5 armor = 34.90%, physical damage is cut to 65.1%
62.67% to 65.1 is a 3.88% ((65.1-62.67)/62.67 x 100%) increase in physical damage, for the same amount of rating as 3.33% crit, and IF 1% crit = 1% more damage in the long run, then the armor pen is 16.5% ((3.88-3.33)/3.33 x 100%) MORE effective in this case, which is very possible as its 160ish rating for both, and in pvp this armor amount is a bit higher than a hunter. BUT *disclaimer* armor penetration effects only physical damage, and crit could also proc other things to increase dps.
However since it's a bit sweeter to remove the last part of an enemies armor than the first part, if you continue to pile on armor pen to ignore 50% armor (4537.5 armor, 22.95% physical damage reduc), which is maybe doable with trinket procs and gems, you get 22.95% ((77.05-62.67)/62.67 x 100%) more physical damage, which is 37.76% more effective than the corresponding 16.66% crit, again if 1% crit was equal to 1% more physical damage.
Another point is that stats work together. For example if you had 20% crit, then lets say your damage would be 120% (100% = your damage without crits + 20% if crits are 1% straight more damage). If you add 3.33% crit, then your damage is 123.33%, which is actually a 2.78% ((123.33-120)/120 x 100%) increase over where you were at, whereas if all your damage was physical, 10% armor penetration, as shown above, would multiply with, instead of just add to, your 120% damage for a 3.88% damage increase, 39.57% ((3.88-2.78)/2.78) x 100%, OMG) more effective than the crit assuming everything i assumed to crit before to make these calculations simpler, and at this point and if you pile on armor pen even further, armor pen should definitely outweigh the fact that it probably doesn't affect as many things or increase uptime of talents and abilities like crit.
So basically armor penetration seems pretty good now, which i even just convinced myself after typing all this, and should scale better than other stats in high amounts and when the enemy has high armor (or admittedly less effective against low armor targets). Even though its new, I think its not trash, and therefore should not gtfo off physical damage gear like haste should.