I take a different approach than Chronalis does and it works for me. I've always been a fan of high avoidance so i tend to gear that way, sometimes at the expense of stam. My goal is to reach just enough health to be able to survive a given boss fight and after that it's avoidance, avoidance, avoidance.
When i first tanked Naxx25 at 80 I was at 539 defense and didn't have much over 20k health unbuffed, so don't let anyone tell you it's impossible, you just need a good team backing you up. I've now cleared every encounter in the game so far (with the exception of Sartharion with all 3 drakes up) as MT. My guild let me start MT duties at 80 because I had been with them all the way through Sunwell so they gave me a shot. Since then I've been praised by dps for my insane threat and by healers for how easy it is to heal me (and this is compared to 2 ferals and a warrior tank we also have in guild). The simple fact of the matter is, healers appreciate you taking less damage no matter how large your health pool is.
Currently my unbuffed stats are as follows:
Health: 26344
Miss: 10.68% (This won't be on your character sheet. I would recommend the Tankadin mod.)
Dodge: 22.07%
Parry: 18.16%
Block: 18.43%
BV: 1183
Hit: 160
Expertise: 13
I tend to socket mostly for Defense and use every Defense enchant available in the game. This will not only raise all your avoidance stats across the board instead of just just one (which is a huge concern with diminishing returns on avoidance now), but the more defense you have the larger the miss % is as well. I honestly wouldn't focus too much on hit or expertise, but as Chronalis said, don't automatically pass up a piece of gear with lesser stats elsewhere but with hit, it's always good to maintain a balance and to also develop multiple sets of gear for different purposes.
Feel free to check out my amory here.