Ret Pallies and Armor Pen and Deathbringer's Will
Let me start by saying I am in no way excusing a ret for rolling on it and had to argue with one to not roll in a guild cooperation run recently. Having said that, it could be a lack of trinket upgrades available for ret.
Mirror of truth/corin's cromium coaster
Mark of supremacy
If you weren't in a raiding guild during uld/toc that could do 25s, that's it unless you can afford Greatness and the iLvl 200 makes it harder to get into pugs because they don't inspect. Lots just can't afford Greatness anyways. Pyrite infuser drops frequently off of FL during the weekly but the hit is often wasted (as is the hit from mark of sup.) more than anything, I thinks it's a lack of practically available options unless this is an older toon or in a 25s guild willing to run older content. ArP is just so prevalent on gear now and so many other specs stack it, the stat is likely confusing as well. Heck, it's so prevalent that it is even on some of the ret BiS gear.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng