75% is the maximum reduction that ARMOR will provide. Which isn't a big deal for us because it's not likely that we'll be able to stack enough armor to reach that point. Death Knights and (especially) Druids have abilities that increase their armor by a great deal and it is something that may be of concern to them.
Other -% mitigation like Sanctuary, Righteous Fury, etc are still applicable, but they don't add straight onto your armor mitigation.
Armor = 62%
Sanc = 3%
Righeous Fury = 6%
Shield of Templar = 3%
You do NOT have 62 + 3 + 6 + 3 = 74% damage reduction. That's not the way they stack. First 62% of the total damage would be reduced, and then the damage reduction from talents would be applied to what's left (the remaining 38% of the damage that armor could not mitigate). So while yes, in theory you could have more than 75% total mitigation using talents + armor it's not so likely that a Paladin will ever have to worry about it.
Also blocks, shields, and anything that shaves a fixed amount of damage off an attack can apply too, to the point that you COULD have 100% theoretical mitigation.
Consider this sample using EXTREMELY simple conditions:
You are Protection spec but you are NOT tanking. You're along for the ride, and have Righteous Fury and Sanctuary turned off. Your armor provides you 60% mitigation. Shield of the Templar provides you 3%. This is designed to make the situation extremely simple.
You are hit, you do not block, for 10000 physical damage. Your armor reduces this damage by 60%, leaving 4000 physical damage. Shield of the Templar reduces this remaining damage by 3%, leaving 3880.
Mitigation by armor is something that we should not be highly concerned about. If we gear up properly in Plate in such a way that we have all of our other defensive stats covered, we should have an appropriate amount of armor.