Here's material from an earlier post I did on +Defense Gear...

This post is about gear choices for warriors and represents the path I’ve followed. This post is about tanking gear choices. I’ll do another one later about FR gear choices.

+Defense FTW

The big thing against the instance bosses is to avoid being taken out by their massive critical strikes. For a level 60 player with 300 defense against a level 60 mob (level*5 = 300 weapon skill), the mob has a 5% chance to critically hit the player. For many bosses, a critical strike is 2,000+ damage. Two in a row will take down almost anyone that isn’t at full health. Any creature 3 levels higher than you will land crushing blows. At level 60, you will take crushing blows from Molten Destroyers, Lava Reavers, and boss creatures in Molten Core.
Defense is the skill that helps you avoid damage from your opponents. The official definition from Blizzard is:

• Increases the chance of being missed by an attack.
• Increases the chance to dodge, parry, and block.
• Decreases the chance of being affected by a critical hit.
• Decreases the chance of being affected by a "crushing blow".
• Creatures that are higher level than your character can land crushing blows that deal increased melee damage. The chance of a crushing blow increases as the level difference between you and the opposing creature increases. Players never deal "crushing blows", only creatures.

In practice, equipping an item with +defense that one point of defense skill increases your chance to dodge by 0.04%, chance to block by 0.04%, and chance to parry by 0.04%. It has been stated by Blizzard that a point of defense reduces your chance to be critically hit by 0.04% - though your chance to be critically hit is relative to your opponent's weapon skill (and any +crit items they are wearing, of course).

The real benefit to defense skill is the reduction in critical hits. You can't dodge or parry every attack, but you can make it so you take virtually no critical hits in combat. It's generally spike damage - multiple critical hits and crushing blows - that kill tanks and overwhelm the ability of healers to keep you up.

How much defense is enough?

If you want to absolutely minimize your chance of being critically hit, the number is 440 defense skill - that will reduce the critical chance of any mob, boss or not, to effectively zero. At 425 defense, the chance that you will take a critical hit is less than 1% against anything you will encounter in the game right now, and will remain less than 1% even against level 64 mobs. Here’s the problem…you can't easily get 425 without going into green gear, or without having some very nice gear. At 365 defense, you have about a 3% chance of being critically hit by a level 63 mob, and 2.6% for level 60 mobs.

When is +defense worth getting?

I wouldn’t worry about it until you are at (or very near) level 60. Most +defense items are pre-MC end game. In my case, I worked on my shopping list from level 58 or so onward. I would always pick up a Valor set piece if I could, but that was an interim choice until I could get what I wanted.

Beep’s +Defense List

I will acknowledge my debt to Evergreen’s popular WoW forum post on this topic. That post has over 70,000 views to date. Here’s the list of gear I had as we began tackling MC:

• Head – [ITEM]Enchanted Thorium Helm[/ITEM] - 526AC, 12 str, 25 sta, 9 def. Crafted Item.
• Neck – [ITEM]Medallion of Grand Marshal Morris[/ITEM] - 7 sta, 10 def. World Drop.
• Back - [ITEM]Cloak of Warding[/ITEM]- 214AC, 5 def (with +70 AC enchant). Crafted item.
• Shoulders – [ITEM]Stockade Pauldrons[/ITEM] - 472AC, 20 sta, 10 spr, 10 def. World Drop.
• Legs – [ITEM]Enchanted Thorium Leggings[/ITEM] - 575AC, 20 str, 21 sta, 8 def. Crafted item.
• Chest – [ITEM]Ornate Adamantium Breastplate[/ITEM] - 807AC, 22 sta, 10 def. Quest reward.
• Bracers – [ITEM]Vigorsteel Vambraces[/ITEM] - 283 AC, 7 agi, 17 sta, (No Def…) +3 Def Enchant. Drop from Darkmaster Gandling in Scholo.
• Gloves – [ITEM]Stonegrip Gauntlets[/ITEM] - 392AC, 9 str, 14 sta, 10 def. World Drop.
• Belt – [ITEM]Belt of Might[/ITEM] – 421AC, 21 str, 15 sta, 7 FR, 5 def. Trash Mob Drop (MC).
• Boots – [ITEM]Crystal Encrusted Greaves[/ITEM] – 512 AC, 21 sta, 5 def. Quest Reward.
• Shield – [ITEM]Draconian Deflector[/ITEM] - 2153AC, 7 sta, 10 def. Drop from Drakkisath (UBRS).
• Trinket – [ITEM]Force of Will[/ITEM] – 7 defense. Drop from Emperor in BRD.

While I didn't have this, the [item]Deathbone Chestplate[/item] is also good with a huge +17 Def that can boosted another +3 def with a two-piece set bonus.

Base Defense Skill = 300
Anticipation Talent (Protection Tree Talent) = 10
Defense from items = 92
Total = 402
All these items except the Belt of Might are available BEFORE Molten Core, as quest rewards, instance drops or world drops (usually easiest to purchase at the AH). There are other choices of items that have +defense. The key is to plan and keep an eye out for what you want.