It could be interesting to notice if talents like survival of the fitness (feral druid, +6% all stats) or Blessing of Kings once multiplied by the base agility are affected by diminushing returns or not (aka, 87 base agility +SoF+BoK = 101 agility, 101-87 = 14, are those 14 agility affected by diminushing returns?).

Now we have everything we need, we're ready to do some multi stat calculations.

Current Stats:
Level 60 warrior
Base defense: 300
Base dodge%: 7.0076994895935
Base parry%: 5
Dodge rating: 0
Parry rating: 0
Base Agility: 83

Put on some gear which gives
+8 Defense
+24 Dodge Rating
+20 Parry Rating
+21 Agility

How much Dodge% and Parry% do I have now?


Solution:
For a level 60 Warrior
1 Defense = 0.04% Dodge + 0.04% Parry (the rest are not important for this problem)
12 Dodge Rating = 1% Dodge
15 Parry Rating = 1% Parry
1 Agility = 0.0427% Dodge




Dodge Cap
Parry Cap




Add the base values and we have:
Dodge = 10.2487108
Parry = 6.668054461


Answer: (These are the values taken in game)
Dodge = 10.248710632324
Parry = 6.6680541038513

Does this mean, that miss chance from defense is not subject to diminishing returns?

Miss is very likely subject to diminishing returns. But unlike dodge and parry there is no easy function to get the values after diminishing returns.
So we'd need someone getting hit a lot of times at one defense value to determine the cap in the formula (the ) and a lot of times at another defense value to confirm that it uses the same formula.

Okay, based on Whitetooth's results, we can conclude that the % increase in average time-to-live is given by the following expression:



Where h is the chance to be hit before the generalized avoidance factor a is accounted for, a_c is the cap, n is the constant in the neighborhood of .956%, and a_u is the avoidance above base before diminishing returns, and a_0 is the base avoidance.

As increases, this quantity increases, indicating faster-than-exponential growth. So, in my mind, Blizzard has still not come close to fixing the problem. Avoidance is still better the more you stack it, particularly since the different avoidance stats don't share DRs. The only thing this will accomplish is encouraging tanks to stack a variety of avoidance stats, which for some is impossible.

They might, but they can dodge, parry and be missed like warriors so they don't really need a higher dodge cap like druids.
Some data to evaluate it will help though. With the formula known now few data points are enough to determine the caps. My guess is that they will be equal or close to the warrior values.

Deathknights should be interesting though. With their strength to parry conversion they might have a higher parry and a lower dodge cap.

Valid Datapoints should look like? I can give you some with my lvl 70 Paladin.

LVL70 Bloodelf Paladin
AGI Defense Dodge R. Dodge % Parry R. Parry %
79 350 0 6,45 0 5
79 366 44 9,45 0 5,66
79 371 74 11,16 0 5,86
79 379 97 12,57 0 6,18
87 403 97 13,72 0 7,12
92 412 97 14,21 0 7,46
109 429 131 16,87 0 8,09
109 502 288 25,03 75 13,02

No idea how to format it, sry.

Use the table tag:

LVL70 Bloodelf Paladin

AGI	Defense	Dodge R.	Dodge %	Parry R.	Parry %
79	350	0	6.45	0	5
79	366	44	9.45	0	5.66
79	371	74	11.16	0	5.86
79	379	97	12.57	0	6.18
87	403	97	13.72	0	7.12
92	412	97	14.21	0	7.46
109	429	131	16.87	0	8.09
109	502	288	25.03	75	13.02

For those that would like to contribute with data, may I suggest using the following command I formated for obtaining more precise data(because 2 decimal places isn't really useful):

/print select(1, UnitDefense("player"))+select(2, UnitDefense("player")).." "..GetDodgeChance().." "..GetParryChance().." "..GetCombatRating(CR_DODGE).." "..GetCombatRating(CR_PARRY).." "..select(1, UnitStat("player", 2))

This will print your stats in your chat frame, that looks like this:
"274 4.1817655563354 6.6373481750488 0 33 92"
Which is Defense, Dodge%, Parry%, Dodge Rating, Parry Rating, Agility in that order.

The /print command is provided by AceConsole-2.0, if you have any ace2 addons you can use it.
Another helpful addon is Chatter, it gives you a button at the bottom right corner of your chat frame click on it to open a window that you can select text and use ctrl+c to copy out your test results.

And if possible, use gear with only 1 DR stat, like pure agi(no defense dodge parry) or pure defense (no agi dodge parry, but block is fine), it simplifies the calculations quite a bit.

This is how I would do it:
1. Remove all equipped items, and reset talents to 0/0/0
2. Find some pure defense gear, no agi, dodge or parry, but block is fine.
3. Put them on one by one and print the data with the command.
4. Remove all equipped items, find some pure agi gear, take the data
5. Remove all equipped items, find some pure dodge gear, take the data
6. Remove all equipped items, find some pure parry gear, take the data

You can find how to use it here: Elitist Jerks - vB Code List