you know that you compare lvl 80 (40% crit) with lvl 85 (10% crit) ?
so do you really think that tanks will have 25% dodge + 20% parry + 80% block with the start of the new expansion?
think again :P
Mastery, for example, is 157 Mastery Rating for 1 Mastery point. That is WAY more than at 80 (Given last I checked, it was like 1 Mastery was like barely 50 Mastery Rating).
And assume obvious other combat rating changes (Dodge/Parry/Hit/Expertise/Crit/Haste) to reflect your level, just like TBC, just like WotLK.
hmmm,
ok what stats will be realistic at 85? Only the first tier set get around 700 mastery with some more epics. This will be 13,5% Block, all 57,5% Block. You want to tell that we will never come over around 37,5% dodge an parry? (miss is provide)
part of the issues that they had in BT, SWP, ToC, ICC and RS was that tank avoidance was shot through the roof and into near-azeroth orbit. tanks were sporting upwards of 60% avoidance. to ensure that tank damage keeps increasing, this means that bosses need to hit harder and harder, or need to use abilities that counteract dodge/parry/block.
recent examples of this were gormok the impaler (dot+ hits hard), anub'arak MT (the ice cleave stuns), sindragosa (frost breath and aura) and lich king (hard hitting + soul reaper). in TBC, many bosses, especially in SWP, had either a strong magic attack (kalec, felmyst), a stacking debuff (brutallus, m'uru) or didn't even melee (eredar twins?, kil'jaeden).
ultimately, the cause of both damage stacking were based on the extra raid tiers blizzard added. for example, SWP was added as a kind of filler, like halion recently. in WotLK, hardmodes were the culprit. in both cases, blizzard ended up adding a buff/debuff that reduced dodge by 20%. this lead to tanks stacking STAM and armor, and ultimately healers needing to spam cast heals. this lead to 3 major problems:
1) healers didn't have the time to decide which heals to use, and were basically spam casting their biggest heal, because the tank could be dead if you healed reactively.
2) tanks ignored all stats for armor and stamina, which means they were a constraint on healer mana.
3) ultimately, paladin tanks proved to be the strongest tank healers, to the point where you were severely nerfed when you didn't have a paladin healer. this was because paladins didn't have mana problems and could basically spam cast 20K HL bombs while stacking int.
now, for cataclysm (and most likely beyond), blizzard wants to achieve 3 things:
1) equalize stat discrepancies where X is miles better than Y (as was the case with stam/armor compared to avoidance).
2) make healer mana matter (by nerfing regen and giving every healer either new tools or a new way to use old tools).
3) make every tank just as good for any job.
all 3 of these have their roots in the new tanks design.
1) avoidance has been equalized: dodge and parry are of equal strength.
2) if healers can heal reactively instead of proactively (as is the current case), they have more time for raid awareness. this means that tanks get more value from avoidance, since an avoided hit often means saved mana for the healer.
3) if dodge and parry are nerfed significantly (halved or more), then death knights (and to some extent druids) don't take massive amounts of damage anymore compared to warriors and paladins (who also had block to reduce some of the damage taken).
overall, you can expect avoidance at T11 content to be around 25% (10% parry/dodge and 5% miss), where every tank also has a method of reducing the damage they take through a kind of block/absorb mechanic. we can expect this % to go up by 6 per tier. assuming 4 tiers (like in LK), that means avoidance numbers end up around 49%, about what you used to have in ICC with chill of the throne. this means that boss damage can scale much slower and, more importantly, avoidance still becomes useful to have (since you get more return on investment from each % avoidance).
TL;DR:
avoidance will be 25% at the start of T11 and around 50% at T14.