For Icecrown Citadel, we are implementing a spell that will affect every enemy creature in the raid. The spell, called Chill of the Throne, will allow creatures to ignore 20% of the dodge chance of their melee targets. So if a raid's main tank had 30% dodge normally, in Icecrown Citadel they will effectively have 10%.

Why are we doing this?

The high levels of tank avoidance players have obtained is making the incoming damage a tank DOES take more "spiky" than is healthy for raiding. Ideally, tanks would be receiving a relatively constant stream of damage over time. This allows healers to better plan their healing strategy, broaden their spell options, and simply give more time to react. Tanks could use their cooldowns more reactively. Instead, the current situation is that if we make a hard hitting melee boss and a tank doesn't avoid two successive swings then the tank could very well be dead in that 1-2 second window. The use of reactive defensive abilities instead becomes a methodically planned affair, healers have to spam their largest heals just in case the huge damage spike happens.

We've been trying to do a fair amount to mitigate the effect of high tank avoidance on the encounter side of things during this expansion with faster melee swings, additional melee strikes, dual wielding, narrowing the normal variance of melee swing damage, and various other tricks. There's a limit to what we can do, however. So to give us a bit of breathing room we’ve implemented Chill of the Throne. Going forward past Icecrown Citadel, we have plans to keep tank avoidance from growing so high again.

We'll have this on the PTR soon so players can see the effects inside Icecrown Raid.

Our original estimations for tank avoidance would have worked fine had we not decided to add extra tiers of gear to reward heroic boss kills halfway through the expansion.

The Cataclysm design will keep tank avoidance at more manageable levels. The loss of defense skill counts for a lot right there. We are also considering giving bosses expertise or other ways of baking in Icewell Radiance -- basically the concept that bosses scale with gear rather than just hitting harder and taking more hits.

It would still be fine if the itemization team had designed the gear accordingly. In a full 258 setup for warrior tanks, precisely two pieces have anything but a 3 way split of pure avoidance stats on them. There's 3 different avoidance stats on 3 different diminishing returns, and pumping them all up like that can really make avoidance numbers go way out of whack. Meanwhile, we lose out on things like Expertise, and the preciously rare Hit Rating which is available on *1* piece of 258 tanking gear and end up having to swap gear around to cover those deficiencies.
You are making the common mistake in thinking that our goal for itemization is to give you the best possible gear that we can. Itemizing your character is supposed to be a choice. There will be better pieces and worse pieces. There will be pieces that combine stats your really want with stats you don't really need. Wearing the best gear for their character (which is not the same as wearing the best gear) is one way players have to demonstrate mastery of the game.

This is also why I always preach to take BiS lists with a grain of salt. Merely reaching for the item declared to be BiS by a spreadsheet or system you might not even understand could lead you to making bad gear choices, often of the variety of passing over the good upgrade because it's not the best possible upgrade.

Also, if you're going to give mobs expertise, can you please make a spell or some kind of method to determine the level of expertise without us having to do parses?
Yes. We would probably just let you see the numbers directly. I consider it a design flaw that players have to experiment to determine thinks like hit and expertise caps. We're all for experimentation and theorycrafting, but we don't think it's fair to require some players to go out and do a lot of work to generate specific numbers that all players feel like they need to know.

Putting so much avoidance on gear isn't a bad idea because other stats are better. It was a bad idea because it causes tank scaling to fail and makes Radiance necessary.
That logic doesn't really work. It's like saying instead of nerfing armor pen, we should have just put less and less on higher level gear.

If we had avoided avoidance on tank gear, then every piece of tank gear would have hit and expertise (and maybe crit, haste and armor pen). Stamina and armor are static amounts, and if they were not, then those pieces immediately become the only pieces players would pay attention to.

If you want ICC damage to be steadier, why don't you just walk over to the item team and say "Hey, we'd like less avoidance, can you cut out half of the avoidance from the ICC gear and replace it with stamina?" Or if you're worried people will get too much stamina, make it Frost Resistance and put in so much Frost damage you couldn't hope to survive long with TotGC gear alone.

We just don't think that works. If you put very unattractive stats on gear then players just go back the previous tier of gear and complain that we don't know how to itemize. If you put bonus stamina on the tier 10 gear, then that means the next tier of gear better have bonus stamina as well. If it has avoidance instead of that bonus stamina, tanks just shrug and go back to the tier 10 gear.

This is not a tank only problem. Casters won't upgrade to gear that doesn't have more spell power on it, because spell power tends to trump everything else for purposes of their dps or healing.

We put a little bonus armor on non-armor items (necks, rings, trinkets and the occasional cloak). We don't put bonus armor on gloves and chests because that gear would be too good.

It's an item level problem. If we added another raid tier to Lich King, we couldn't just keep avoiding avoidance and avoid it for every tier going forward. We just need a system where you avoid a Naxx boss 30% of the time and an Icecrown boss 30% of the time, the same way the Icecrown bosses have e.g. 30% larger health bars and thus take 30% more damage to kill. Otherwise the stats don't scale and bad thing happen (in this case the boss having to land so much damage to account for the fact that it misses so often).