Updated: Patch 3.0 changed everything; see here - Crit Immunity Redux

Let me start this with a bold statement: The single most important statistic that all tanking Druids must attain is 2.6% reduction to your chance to be critically hit by enemies. (Assuming 3/3 in Survival of the Fittest, which happens to be a required talent for all feral tanks.) No matter how much armor, how high your +dodge, and how many hit points you have amassed, if you have not attained crit immunity, then you are not doing your job.

What's the big deal with crit immunity?
While you may have amazing statistics and the ability to take a massive pounding, even allowing enemies a sliver of chance to critically hit you can have serious consequences when you are raiding. If nothing else, Murphy's Law states that you are going to take a big crit at exactly the wrong time if you allow the possibility to remain open - so why take the risk?

Besides your own survival, part of your job as a tank is to make things as easy as possible on your healers. Priests, and other healers, appreciate the feral Druid's ability to get crit immune. Kirk puts it nicely when discussing the benefits of a feral tank:

What this means as healer is that you are far less worried that a sudden hit (much less a crit) is going to plunge the tank’s health into (or through the bottom of) the red zone. Damage is smooth, not spiky.

Removing critical hits from the list of healer worry allows them to get a feel for the amount of damage that you are going to take, and over time they will learn when they can leave you without a heal for a few seconds in order to patch up other party members, and when they need to really focus on you. The random nature of critical hits makes this level of synergy far less possible.

Okay, so how do I get crit immune?
There are two stats that you can accumulate that reduce your chances of being critically hit: defense and resilience. The benefits of these stats (to a Druid tank) at level 70 (for PVE) are:

  • 60 defense rating = -1% crit, +1% miss, +1% dodge
  • 39.4 resilience rating = -1% crit, -2% crit damage

If you were to attain crit immunity using only one stat or the other, this would equate to either 156 defense rating (which translates to 415 defense on your character sheet) or 103 resilience.

Starting out
By far the easiest way for a young feral druid to get crit immunity is by stacking defense rating. Acquire the Heavy Clefthoof set (Boots, Leggings, Vest) in a hurry, either through the Auction House, or from a friendly leather worker. Socket those pieces with +defense rating gems - Enduring Talasite is an easy-to-obtain favorite. Now see how much more defense rating you need, and browse WoW Loot to flesh out your set. It is not to hard to secure immunity following this path.

Mixing and matching
Once you have obtained crit immunity you are going to quickly realize that tying up so many slots on defense rating gear hinders your progression. Now is the time to introduce resilience gear.

It is important to realize that, although 39.4 resilience provides the same reduction in crit chance as 60 defense rating, you lose 1% dodge and 1% miss in the process. To make these up you need to ensure that you add gear that grants these benefits along with the desired resilience. Agility is the stat that you will have to use to cover the difference:

  • 14.7 agility = +1% dodge, +29.4 armor

It is generally accepted that +2% dodge is equivalent to +1% miss and +1% dodge, so 29.4 agility in addition to the 39.4 resilience is equivalent (in fact, superior due to the armor increase) to 60 defense rating.

How you proceed from here is up to you. It is rare that you will find a single piece of equipment that has resilience and agility in the exact ratio that is required to replace the defense on another single piece of equipment. You will have to mix and match. Perhaps the defense on your bracers can be made up with by the resilience on a new cloak, so you will have to find new bracers that compliment the cloak.

Be careful when swapping out defense rating - it is probably worth it to break out a pen and paper and do some number crunching before committing to any switches. In addition, it is recommended that you be a packrat, and save any high defense rating gear you swap out, in case you ever need to juggle while upgrading another slot.

A word of thanks
I just wanted to extend a big 'Thank you' to Wysperra of Draenor and Obara of Muradin whose emails inspired me to get off my lazy tail and write this article. Much appreciated guys!

Sources
WoWWiki - Comabt Rating System
Keith Devens.com
Emmerald's Druid forums