The only real problem I see with Pallies using the same gear as warriors and Death Knights is that unlike those classes INT will still be somewhat relevant for sizing their mana pools.

I am not worried about Prot because of all the healing that they get in raids with stacking stamina will mean they have large health pools to heal back and as such will be getting plenty of mana back for tanking

Ret is a bit of a different story because they won't have as much stamina so less healing on them doesn't grant the benefit of healing for mana, but as I recall in ret there is a talent that returns mana after you judge a seal so depending on how the mechanics of that turn out mana could cease to be an issue for Ret Pallies as well.

Holy paladins will be the only plate class that has their own gear as it will certainly have to key spellpower as a raw stat on the gear. Mana stats like INT and SPI will also be needed on said gear making the Holy Pally a niche with little to no overlap and usefulness to any other class.

That being said I am really happy that I have chosen to quit holy and respec to ret when the expansion comes out.