540 def? Way too easy! I can get you there before you hit 80

Defense rating is the most important stat for any tank. Def does two things, it makes you crit immune and adds to your overall avoidance stats like dodge, parry and block. 540 def rating is the the minimum to be crit immune from raid bosses three levels higher than you are. I say minimum because I hear the words "def cap" get thrown all over the place. I tell you right now, there is no such thing as a def cap, only a minimum of 540, you can go higher if you wanted to. Going higher will add more to your other avoidance stats also, so being a little over won't hurt at all. So, no matter what, defense is the first stat you will work on and every piece of new gear will affect your def rating in some way and you will either have to make up the difference or subtract a few points to add more to another stat.

When I hit 80 I was able to tank VoA 10 man and UK Heroic within 15 mins of dinging 80. All the gear I had was received from quests, world drops, factions or crafted epics which can be obtained with out farming instances for just one drop. Once you hit the magic 540 or get very close you should start going for pieces with more stamina or hit rating and less def since we will make up the difference with enchants. Here is a pre-heroic gear guide on how to do the same thing:

Helm:
[Tempered Titansteel Helm] - Crafted (blacksmithing) BOE, can be bought in AH
[Armored Titanium Goggles] - Crafted (engineering) BOP, can only be worn by engineers
[The Crusader's Resolution] - Quest: The Crusader's Pinnacle, Icecrown
[Greathelm of the Unyielding Bull] -World drop, can be bought in AH
[Greathelm of the Titan Protectorate] - Quest: Reclamation, Sholazar Basin

Neck:
[Titanium Earthguard Chain] - Crafted (jewelcrafting) BOE, can be bought in AH
[Medallion of the Disgraced] - Zone drop from 10 man Naxxramas , can be bought in AH
[Burning Skull Pendant] - Zone drop from Gundrak, can be bought in AH
[Betrayer's Choker] - Quest: Betrayal
[Blood-Infused Pendant] -Quest: A Hero's Burden

Shoulders:
[Wapach's Spaulders of Solidarity] - World drop, can be bought in AH
[Crusader's Square Pauldrons] - Zone drop from COT4, can be bought in AH
[Pauldrons of Reconnaissance] - Quest: Halls of Stone
[Bone-Polished Iceplate Shoulders] - Quest: Hour of the Worg, Grizzly Hills
[Tundra Pauldrons] - Quest: Have They No Shame?, The Nexus

Cloak:
[Cloak of Peaceful Resolutions] - Reputation, The Wyrmrest Accord (Honored)
[Tattered Castle Drape] - Zone drop from Utgarde Keep, can be bought in AH
[Bloodbane's Resolve] - Quest: The Fate of Bloodbane

Chest:
[Breastplate of the Solemn Council] - Reputation, The Wyrmrest Accord (Revered)
[Silver-Plated Battlechest] - Quest: Junk in My Trunk, Utgarde Pinnacle
[Ivory-Reinforced Chestguard] - Reputation, Kalu'ak (Honored)
[Clutch of the Storm Giant] - Quest: The Storm King's Vengeance, Zul'Drak

Wrists:
[Minion Bracers] - Zone Drop from Naxxramas, can be bought in AH
[Bracers of Reverence] - Quest: A Wing and a Prayer, The Oculus
[Tempered Saronite Bracers] - Crafted (blacksmithing) BOE, can be bought in AH

Hands:
[Gauntlets of the Master] - Reputation, Kirin Tor (Exalted)
[Refined Ore Gloves] - Zone Drop from Halls of Stone, can be bought in AH
[Daunting Handguards] - Crafted (blacksmithing) BOE, can be bought in AH
[Giant-Sized Gauntlets] - Reputation, Frenzyheart Tribe (Revered)

Waist:
[Waistguard of the Risen Knight] - Zone Drop from Drak'Tharon Keep, can be bought in AH
[Tempered Saronite Belt] - Crafted (blacksmithing) BOE, can be bought in AH

Legs:
[Special Issue Legplates] - Reputation, Argent Crusade (Honored)
[Daunting Legplates] - Crafted (blacksmithing) BOE, can be bought in AH
[Iron Colossus Legplates] - Quest: The Iron Colossus, Storm Peaks

Feet:
[Tempered Titansteel Treads] - Crafted (blacksmithing) BOE, can be bought in AH
[Sabatons of Draconic Vigor] - Reputation, The Wyrmrest Accord (Revered)
[Toxin-Tempered Sabatons] - Reputation, Knights of the Ebon Blade (Honored)

Weapons:
[Teldrassil Protector] - Argent Tournament, 25 Champion Seals
[Crescent of Brooding Fury] - Quest: Wanted: Ragemane's Flipper, Zul'Drak
[Hammer of Quiet Mourning] - Quest: Wanted: Ragemane's Flipper, Zul'Drak
[Blade of the Empty Void] - Quest: Tirion's Gambit, Icecrown

Shields:
[Titansteel Shield Wall] - Crafted (blacksmithing) BOE, can be bought in AH
[Saronite Bulwark] - Crafted (blacksmithing) BOE, can be bought in AH
[Shield of the Lion-hearted] - Reputation, Alliance Vanguard (Revered)
[Bulwark of the Tormented God] - Quest: Ursoc, The Bear God, Grizzly Hills

Rings:
[Signet of the Accord] - The Obsidian Sanctum from Sartharion BOE, can be found in AH (very pricey)
[Ring of Earthen Might] - Crafted (jewelcrafting) BOE, can be bought in AH
[Staunch Signet] - Quest: The Struggle Persists, The Oculus
[Solid Platinum Band] - Quest: For Posterity, Gundrak
[Kurzel's Warband] - Quest: Search and Rescue, Drak'Tharon Keep

Trinkets:
Hard to find before running heroics, use the trinkets you have already. Anything with stam, str, or agl. An avoidance trinket would be best.

Some BOP profession only trinkets:
[Figurine - Monarch Crab] - Jewelcrafting
[Figurine - Ruby Hare]- Jewelcrafting
[Sonic Booster] - Engineering
[Indestructible Alchemist's Stone] - Alchemy

Libram:
[Venture Co. Libram of Protection] - 30 [Venture Coin] Grizzly Hills

There you go, an easy list of good def gear, all obtainable as you are leveling so when you ding 80 you will be set to run heroics right away. Any thoughts on the list or an item I may have missed, let me know in the comments.