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:
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?

Actually its the same for avoidance too.

2% dodge is worth WAY MORE when you have very high total avoidance (parry+dodge+miss).
The more avoidance you got the more worth avoidance upgrades become.
Think going from 30% avoidance to 35% avoidance vs going from 70% avoidance to 75% avoidance.

NOTE: This is not a post about the value of an avoidance setup (this has been done to death lately in these forums). Just a remark, that the Mitigation mechanic is special in a very certain kind of way.

Got to agree with this, it might equal out to the same thing over time, but the potential to get unlucky with the current system for resistances can be really annoying at times... Especially when an encounter relies on resistances.

I would actually argue the opposite, Blizzard did a horrible job of balancing armor.

Just because the two systems don't work the same way doesn't mean the end result isn't the same. Each side (resists/armor) can be tweaked (as I'm sure they were early on) to produce the same resist/avoidance/reduction rates for spells and physical attacks over the long haul. Now, binary versus partial resist spells seem to throw a wrench in single occurrence situations, I think because they were designed to produce the same effect over a period of time.

You guys aren't really considering a huge factor here.

Sure resistance seems to have increasing returns.

However, there are 5 schools of magic(4 main ones) and each has it's own resist tree.

Armor reduces ALL physical damage.

In addition to this, armor comes naturally on all gear based upon the armor type, the Ilvl of the item and the quality of the item. It is in essense a "free" stat.

Resistance however takes away from an items other stats. Maxing your resistance makes your character overall less effective, which is why you can max it out. Point per point resistance is relatively expensive, try to put together a 350 resistance set for 75% resist and see how gimp your character becomes.

Wrong end of the cannon, but 5-point mangled rip on high-armor opponents beg to differ.

I get what you mean though. :P