Stamina:
Paladins have less base hp than warriors, on the order of 1000+ IIRC. While (and correct me if I'm wrong) prot paladins can get +6% stamina while prot warriors can only get+5% stamina, that 1% stamina is not going to be 1000HP for any reasonable value of gear. Hell, for 1% stamina to be even 100HP you need 1000 stamina.
Hence, prot warriors (even leaving aside that prot warrior sets will probably have more stamina than prot paladin due to paladins needing more stats) will always have more HP and be superior for fights which depend on surviving "one-shot" type abilities assuming equal armor mitigation (and both can wear plate of the same iLvL, so there should be equal armor mitgation, as they have the same +armor talent).
Equal ability to avoid crits.
Warriors have a more consistent ability to avoid crushings. Paladins with sufficiently high block rating should be able to do the same, provided that Holy Shield is up and Redoubt has procc'd, but warriors don't need to rely on a proc.
Defensive stance grants 10% flat mitigation, vs improved righteous fury 6%. Improved defensive stance grants a further 6% spell damage reduction, spell warding grants a further 4%. Warriors have better passive mitigation.
Equal ability to parry/dodge and to gain further parry dodge, leaving aside again that warrior gear can probably get more due to paladin item budget being spread thinner.
As such, paladins are directly inferior for MITIGATION in every way.
The hope for paladin tanking must lie in the things paladins have that warriors don't:
1) Ardent Defender- honestly seems preferable to rely on superior stamina and mitigation than this, short of a gimmick fight where everyone is constantly semi-Enfeebled or something. Not really a niche.
2) Ranged taunt- this one has definite possibilities- wouldn't it have been amazing to have back in the day on a fight like Sartura (or Noth for that matter)? Bit gimmicky, and not so much an MT thing probably, but such is life.
3) Divine Shield- rarely going to want to do it as the MT, but still an awesome "oh hell" move provided that you have a few seconds before the mob would be able to nuke the raid (especially combined with the ranged taunt on tauntable mobs).
4) Threat output. This is the big one, since it's applicable to most fights whereas the first ones are gimmicks. Can a paladin tank maintain an acceptable level of mitigation but output more threat? For a DPS burn fight, provided it doesn't also stretch the limits of MT survivability, it is very conceivable that the best solution is the tank with the best threat output.
If a paladin cannot out-threat a warrior tank, yeah, no hope for paladins as *MT* on anything non-trivial. If they can, there's potential.
Anyone got data for warrior vs paladin threat output vs the same type of mob? I've had my first run with a paladin tank in SH last night, but he was a) 47/14/0 holy spec and b) woefully undergeared and yet because of that run I have at least some confidence in paladin tanking..
Picture this. "Welcome to level 70, holy paladin. For your first level 70 instance, we will make you tank SH. What o you mean you never tanked before and you've never seen SH before? What do you mean you have only one piece of gear with +defence? Never mind, we'll tell you what to do! Oh, and we had a PuG rogue without improved sap and like level 63 weapons."
Amazingly we one-shotted everything and only one wipe, which doesn't count because it was the rogue screwing up and inadvertently pulling an extra pack of mobs, so it wasn't too bad, and gave me confidence he could at least hold aggro off the healer
