The easiest way to think of DR, is let's say Dodge is a rich fudge, and parry is ice cream. You eat too much dodge(fudge), and it's less effective how happy it makes you - you eat too much ice cream (parry), you get a brain freeze. Ok that's not a great analogy, whatever.

Simply, both stats are on DR, meaning the more you get, the more it takes of them to provide the same results. Simulated numbers for ease:
When you have 200 Dodge Rating, let's say you get 2% dodge - so you'd think ok 100 Dodge Rating = 1%; well this isn't true because of DR, it'll actually take something like 315 to get your next 1%. As you get more dodge, it's going to take more rating to actually increase your dodge chance. The same is true of parry. So to get the most out of your stats, you want them to be as close as possible (save for the ~200 or so parry you'll get from raid buffs); in order for them to be the most effective. The reason being is that, when you have 1600 parry rating for 14% parry, and 800 dodge rating for 7% dodge; you'd get a lot more avoidance out of getting 100 more dodge rating than you would parry. For 200 parry you'd get say .2% parry and for 200 dodge rating you'd get say .6% dodge.

Does that make sense? I'm not sure it does, but that's the easiest way I can explain it without going into nasty maths. There's a few great blog posts on it out there.

As far as Mastery vs dodge/parry, you're talking about two different animals. One is mitigation the other is avoidance. Mitigation is essentially decreasing all damage taken, it's mitigating the amount of damage you're talking regardless, while avoidance is a chance to not take damage (physical vs spell issues aside). Neither of them are technically 'better' than the other since they're different elements. Sure having 90% dodge+parry would be nice (albeit boring), if you had 0% mitigation, you'd get 1-shot 10% of the time. In contrast if you had 90% mitigation and 0% dodge/parry, you'd take rather consistent damage, and likely be unkillable - but healers would have to constantly heal you. The two work in conjunction, but differently. Ultimately in a real world scenario you need both, and it's mathy debate as to which is best in which scenarios (and it technically changes between fights and mechanics). Think of your tank as... well a car, where your suspension (mitigation) evens out the bumps in the road, and your steering (dodge/parry) gives you a chance to avoid those bumps. Both are important for different reasons. Again not the best analogy but I haven't had coffee yet.


P.S. I'm pretty sure parry-haste has been removed completely at this point (pretty much); but maybe paladins have a talent or something. Even so, an additional melee swing doesn't help your overall survivability only threat; and if a paladin is having threat issues, they're doing it wrong.