This ability really should be allowed to cast at anytime. Having to eat a shot when you know enemies are around the corner that one shot is pretty meh.
It would be pointless since it works off a portion of missing HP. They need to rework Iron Skin to provide you a portion of Max HP as Barrier and the skill should be usable at any time. If you are below 50% HP, the Barrier duration and effectiveness should be increased. Ranks of Iron Skin should also reduce the Cooldown of the Skill as well as the effectiveness bringing it inline with Challenging Shout.
Enhanced Iron Skin should be the Iron Warrior Aspect granting Unstoppable and increasing DR for the duration.
Changing it this way makes Offensive and Premptive (for those that use Conceited this is huge) and boosts the reliability and effectiveness while still maintaining that bonus for using it reactively.
I’d like to see a passive benefit ontop of all the obvious changes.
%DR While Injured seems to be low-hanging fruit for a Passive Boost to Iron Skin even when it is on CD. Either that or a better version of Thick Skin which adds Fortify % every time you take direct damage.
The Healing Effect Upgrade of Iron Skin is way too good to pass up. Perhaps if the choice was the Healing Upgrade or rolling Conceited in as the 2nd upgrade, it would make more sense than Fortify. One path is Defensive focused and one is Offensive focused. Running 3 Shouts + Iron Skin, you would likely opt for the Offensive upgrade.
Yeah, I want perma Colossus (x-men ability) status all the time for Barb too! And licensed cosmetics to boot!
POE has an animation like that for their Steelskin skill.
Ironskin should have a passive armor boost. Lots of Druid skills have passive effects. We need this.
this is what happens when blizz gives barbs a crusader skill. CHAOS INSUES
But in fairness, top barbs in pvp have near 0 cool down already so they really dont need the buff…better to buff some AOE damage…WW re-buffed anyeone? 
Uh the only method to reduce your CDs is via Cool Down Reduction Affixes on gear…which have been nerfed. There are certain Skills like Leap which with Perpetual Stomping can be reset with Ground Stomp and Kick, giving you 4 Leaps back to back.
There is nothing special in PVP making cooldowns 0 for Barbarians, or PVE for that matter. Even with max rolls on CDR Affixes, you will have in some cases 10-15 second CDs on the low-end.
iron skin is a pretty short timer compared to shouts. i think maybe steel grasp would be a better choice? I dunno lol.