To be fair, the reason Warlocks can't tank as well is because they do too little threat while also taking a lot more damage. If GBoK really does 1k+ TPS under normal raid circumstances, you can easily gear a Paladin to be more sturdy (outside of cooldowns) than even a Prot Warrior, as even the Prot Warriors will want TPS gear. The big (BIG) problem is taunt, and no Shield Wall - but IF the GBoK spam really is viable then Paladins do theoretically have a useful spot in raids.
Mana isn't necessarily even an issue. Seal of Wisdom + Judgement of Wisdom + fast weapon (since spell damage isn't necessary) will be enough mana for most fights that don't require taunt.
I don't disagree that a Warrior is absolutely a better overall tank - @
Coldfrostzero isn't arguing that either - but this way Prot Paladins DO have a spot in raids. They'd just be geared after the main tank Warriors, but before the Fury Warriors.
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Point out what's wrong about it instead. Yeah, a tank Paladin under normal circumstances (not GBoK spam) is at best a niche tank for dungeons and some AoE fights if you don't want to spend a ton of gold just to do it a bit quicker - and they're basically just significantly worse at everything else than Warriors normally.
But if the GBoK spam really does generate 1k+ TPS, then Prot Paladins do have some potential relevance in raids. GBoK costs 150 mana which, while it isn't nothing, isn't insurmountable either.
EDIT: Another big issue is of course the gear - a ton of the Classic tank gear is locked to Warrior. There's a bit of leather tank gear since Druids were somewhat planned to tank, but there's very little that suits a Paladin from raids, at least those currently available. You'd basically have to be DPS/healer geared just out of lack of options.