The 4.0.1 problems with PvP and the attempt to solve them really highlighted some of the problems with Resilience. When 4.0.1 came out there was a big problem with people getting one-shotted or nearly one-shotted in battlegrounds. I forgot to put on my PvP gear in one Arathi Basin battle and took an Arcane Blast critical for around 28k. Apparently some ret paladins were doing single hits of up to 20k.
Obviously people being able to strip off half your health with a single attack in PvP is a big problem. On the other hand, people being totally unable to do any damage is a problem as well. Their initial reaction to the problem was to increase the effect of Resilience by 50%, so my 40%-ish damage reduction increased to 60%-ish damage reduction. Now that 28k Arcane Blast crit would only be 11k. Without the critical it would be a measly 5.5k.
In fact, I was usually pretty much invincible. I would heal myself through three people attacking me, despite stuns, silences and fears. Of course Blizzard noticed that this resilience chance was too much, so they lowered the effect of Resilience it by 25%1.
But when I go into battlegrounds, I'm still ridiculously tough. Other people on my team still get one-shotted. Why is this happening? It's because PvP balance comes from a single stat that starts at zero and gets better the more points you have of it.
Blizzard has put a lot of formulae in the game - some more complex than others - to try to prevent stats from getting better the more you have. For armor, the damage reduction formula means that armor expands your effective health linearly. For dodge and parry the diminishing returns formula does not quite come up with a linear result, but it was put into place to avoid these stats from becoming too powerful when you stack too much of them. Resilience has no such diminishing returns, it's power is just left to expand unchecked.
The math behind this is really simple. If you get a certain amount of resilience you take 1% less damage from your enemies. So if you started with zero, then you would now be taking 99% damage instead of 100%. But if you started with enough for a 40% reduction, you'd now be taking 59% instead of 60%. The thing is that while in one sense 59% is 1% less than 60%, the actual amount of damage you are taking with the extra resilience is not 1% less than what you were taking before. 59 divided by 60 is 0.983, so you are taking about 1.7% less damage then you were before you added that resilience. Resilience is 70% better when you already have enough for a 40% reduction than it was when you had none at all in terms of helping you live longer.
Making matters worse is that, as shown above, they like to fix PvP problems by adjusting the amount of damage reduction resilience gives. When they increased the value of resilience by 50%, the moved me from 40% to 60% resilience. I now took 40% instead of 60% of the damage my opponent's did. I was taking two-thirds of the damage, and my time to die from the same pressure by 50%.
A very well geared PvPer can get closer to 50% resilience. Increased that by 50% makes it 75%. So they take 25% instead of 50% of the damage they would have without it. They take half damage, and their time to die is doubled. Buffing resilience may make PvP more survivable on the whole, but it also drastically increases the disparity in power levels between the best and worst geared PvPers.
On top of this, so much PvP balance being dependent on resilience really messes up early expansion PvP. There were a lot of complaints about the first arena season in Wrath because people died so fast there was no time to have a good fight. The response from Blizzard was that we should wait until people get some resilience. There was a little more to it than that - people had easy access to relatively high level PvE gear via Naxxramas and much more limited access to PvP gear - but the increasing returns nature of resilience will always mean that PvP works without a certain range of resilience values and breaks down outside that range.
The incredibly high coefficient on resilience in Cataclysm will probably end up keeping resilience damage reduction values very low - unless they decide to improve them, damage reduction from resilience shouldn't get to even 20%, let alone 50%. Because of this, the problems with resilience will be much less pronounced in Cataclysm than they were in Wrath. But bad math continues to be bad math even when numbers close to zero make it not matter much.
The resilience formula should look just like the armor formula so that effective health in PvP expands linearly - rather than asymptotically - with resilience.
1. That is, they lowered it by 25% of the new value, which was 150% of the old value, so it was now 112.5% of the old value. Despite the fact that they gave an example of what they meant, I had to read their post a couple of times to figure out what they had done. Changing current values by percentages is quite uninformative.