Warrior tanks are better than Paladins when the incoming dps is less then around 350-400 dps and there are no huge hits. When the incoming dps is over 400 DPS paladins are better (this is purely based on Defiance + IB spam vs shield oath and does not inclued any of the paladins cooldowns). So pretty much when you include all the damage mitigation CD's that paladins have it leaves warriors only being better tanks in 4 man dungeons ( and normally only when they are over geared for it.)
EDIT: I had done some calculations to prove this but i can't seem to find that post... I'll edit it in if/when i find it. Post added!
Ok lets put it this way... You need 25% more heals than a paladin (not counting wrath stacks... that will be factored in as i go)... if you use inner beast every time its up that puts you at keeping about 7-8%+ healing from wrath stacks - that means your inner beast has to heal for 17-18% of all incoming heals to be equal to paladins 20% damage reduction. So lets take a 20 sec time period ( roughly what it takes to get back to full stacks ) say your inner beast averages 1200 heal. that means in a 20 second period you could only take about 7200( 1200 is roughly 17% of 7200) in heals to break even with what a paladin has from just the 20% reduction. So in other words if you take more than 360 dps(7200 heals needed/20 seconds) your mitigation is worse than a paladin with no CD's used... 360 dps is not very much... If you take less then 360 dps you are better off than a paladin(that uses no tanking cooldowns).
Using an IB average of 1500 inner beast heal would mean the DPS taken would need to be less then 450 dps... Remember this is not counting any of the paladins cooldowns and I think its pretty obvious the paladin ones are better than the ones warrior gets for mitigation.
So basically the threshold for when paladin surpasses warrior in tanking ability is somewhere around 300-400 incoming dps (when considering all cooldowns from both classes). So as incoming damage increases warriors get less and less viable - hints why they are being avoided for Coil content (where i believe the auto attack on bosses alone = around 450-500 dps not counting any special skills).