This has been floating around in my head for a while but I haven't put it in words before. Basically, damage mitigation is biased against spell damage.
As many of you may know, armor doesn't actually suffer from diminishing returns as an increase of X armor will always yield the same additional lifespan (until you're at the 75% cap). For example, to quote a thread from the warrior forums:

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Originally Posted by satrina
1) I have 3000 armour and 5000 hitpoints. Against a level 60 opponent that deals 200 damage per second I will live for 38.64 seconds Now I will add 1000 armour, giving me 4000 armour. Against a level 60 opponent that deals 200 damage per second I will live for 43.18 seconds Adding 1000 armour increased my time to live by 4.54 seconds.
2) I have 6000 armour and 5000 hitpoints. Against a level 60 opponent that deals 200 damage per second I will live for 52.27 seconds Now I will add 1000 armour, giving me 7000 armour. Against a level 60 opponent that deals 200 damage per second I will live for 56.82 seconds Adding 1000 armour increased my time to live by 4.55 seconds.
3) I have 10000 armour and 5000 hitpoints. Against a level 60 opponent that deals 200 damage per second I will live for 70.45 seconds Now I will add 1000 armour, giving me 11000 armour. Against a level 60 opponent that deals 200 damage per second I will live for 75.00 seconds Adding 1000 armour increased my time to live by 4.55 seconds.
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Relevant information to how this is calculated (and the source of the quote) is
here. In my opinion, this shows significant foresight on the part of Blizzard.
However, when you look at the system in place to combat spell damage, the results are quite different. To resist 75% of the damage of an enemy that is level X, you need 5x resistance. Therefore, your average percent resistance against a level 70 enemy is (Resistance * 75 / 350) or (Resistance * 3/14)
Now then, lets do the same thing as Satrina did above.
1) 116.7 resistance, 5000 HP. Against an enemy that deals 200 DPS, you live for 33.33 seconds.
2) 233.3 resistance, 5000 HP. Against an enemy that deals 200 DPS, you live for 50 seconds.
3) 350 resistance, 5000 HP. Against an enemy that deals 200 DPS, you live for 100 seconds.
The first increase of 116.7 armor increased survival duration by 16.67 seconds, while the second increase of 116.7 armor increased survival duration by 50 seconds. Equal increases to resistance increase your survival duration by higher and higher amounts.
As a side note, I also find it annoying that anyone can go to the auction house and pickup random greens "Of X Protection" and get enough resistance to effectively neutralize a specific damage type, and yet the same cannot be done for physical damage.
Anyway, this is probably totally unimportant in the grand scheme of things, but I felt like writing it anyway. Any thoughts?