Warriors and Paladins also get passive 6% magic reductions at all times through talents to match the passive contribution of Magic Suppression.
One value that is hard to measure but can be terribly significant where it matters most is Acclimation for Frost DKs. In situations where there is consistent enough damage to keep it stacked you can be looking at upwards of 40% magic damage reduction against that school, but if the damage is infrequent the average can skew way down.
And Dracunis, damage reduction %'s do not stack additively, it's a sort of inverse multiplicative.
65% reduction from armor
12% reduction from talents
3% reduction from buffs
Total Damage Reduction = (1-0.65)*(1-0.12)*(1-0.03) = 29.876%
Or, they are applied step-wise (same thing just illustrating with real numbers). If you are hit for 60k before mitigation:
60,000 reduced by 65% = 21,000
21,000 reduced by 12% = 18,480
18,480 reduced by 3% = 17,925.6
(17,925.6 / 60,000 = 29.876%)
So the logical extension if you cannot see it is that stacking % damage reductions can never quite fully close the gap to 0% damage taken.
Also, the different effects do not contribute towards the same cap, i.e. Armor is capped independently at 75%, but total reduction is not. In reality, if you factor the contribution of avoidance in total average damage reduction as well, well-geared tanks will be looking at 88-92% total physical damage reduction before even using cooldowns.
Also, by interesting extension, your 50% Divine Protection for Paladins will only actually reduce your physical damage taken by 2-4% from the original unmitigated values, depending on gear, etc. Though it will be a 50% reduction on the numbers you're used to seeing pop up after all your passive mitigation.