Table of Conversions:
2.4 Defense Rating = 1 Defense = .04 Miss/Doge/Parry/Block
20 Defense Rating = 1% Dodge/Parry/Miss (combined)
15 Defense Rating = 1% Dodge/Parry/Miss/Block (combined)
18.9 Dodge Rating = 1% Dodge
23.7 Parry Rating = 1% Parry
7.9 Block Rating = 1% Block
25 Agility = 1% Dodge
(This taken from this thread and edited per Eanin)
General theory principles:
1. Dodge Rating will always be better than Parry Rating for uncrushability due to the rating difference. It is also better than straight agility.
2. Block Rating is largely unnecessary for raid boss tanking after uncrushability has been attained. For trash tanking, block rating is always good, along with block value.
3. Stamina is always a powerful option in gems and enchants because of its effects on effective health.
Gems:
-Solid Star of Elune (12 stamina, Blue): This gem gives the largest bonus to effective health of all gem options prior to the BT/Hyjal gems, which are 15 stamina each and ridiculously expensive to buy. If you are uncrushable by a solid margin, this will give you the most survivability of any gem option.
-Enduring Talasite (4 Defense Rating and 6 Stamina, Blue/Yellow): The best yellow hybrid available, giving both a modicum of avoidance and stamina. This is also a solid gem choice for filling the defense gap in the 2.3 badge/ZA gear. This gem is best used to complete socket bonuses in gear, especially gear that has a stamina socket bonus. Completing a socket bonus for 6 stamina on a 3 socket piece is worth it most of the time, even if you lose 6 stamina for a R/B/Y socket color set (as is the case with a number of good epic tanking pieces). The defense rating is worth ~1.66 defense, which is adjusted based on your total defense, making it either .16 avoidance or .32, based on how much defense rating you have.
-Glowing Nightseye (5 spell damage and 6 stamina, Red/Blue): If you feel slightly low on spell power and have a red socket to fill, this is a good option. Generally, however, this is inferior to Solid Star given that a weapon can give sufficient spell power to hold threat for a long time in terms of raid progression. As with the Enduring Talasite, this is a choice gem for completing socket bonuses.
-Shifting Nightseye (4 agility and 6 stamina, Red/Blue): Another red gem option, giving slightly more avoidance in lieu of threat. The agility is worth .16 dodge.
-Subtle Living Ruby (8 dodge rating, Red): The best straight red gem available for conversion. If you're short avoidance and need a red gem, this is a good gem to choose. Each gem is worth .42 dodge.
-Thick Dawnstone (8 defense rating, Yellow): The only straight yellow gem worth considering whatsoever. The defense rating is equivalent to ~3.33 Defense, which rounds based on your total defense rating. It is approximately either .48 or .64 avoidance.
Meta Gems
-Powerful Earthstorm Diamond (18 stamina and stun resist): Extremely potent for a gem slot After Kings and talents, you're looking at closer to 23 stamina from this gem, for 230 health. Personally, I prefer this meta, especially for it's easy (at least 3 blue gems) requirement. You were going to have 3 blue gems anyway.
-Tenacious Earthstorm Diamond (12 Defense Rating and restore health on hit): 12 Defense Rating = .6% avoidance, .8% for uncrushability, 5 defense. The health restore effect is bonus threat. Requires 5 blue gems, which shouldn't be a stretch in most gear setups. Realistically, this is a solid gem choice as well, but as mentioned above, I prefer the Powerful for its large bonus to health. If you're not yet uncrushable, this is a good gem choice.
-Eternal Earthstorm Diamond (12 Defense Rating and +10% Block Value, 2.4 recipe): 12 Defense Rating = .6% avoidance, .8% for uncrushability, 5 Defense skill. The beauty of this is the block value. For certain gear sets, this might be the best choice for meta, while the requirements are only 2 blues and 1 yellow, which are easily attainable in any tanking gear set. Since it's scales, this gem has the potential to be far more powerful than any other Meta.
General Gem Theory
As Joanadark wrote at Tankspot, gear is more efficiently itemized into mitigation/avoidance/stamina than spell damage, hit, spell hit, etc. In that regard, Solid Star of Elune is the most powerful gem available for overall stats. Consumables are more effective for threat than survival, though, so if you're having issues with that, then look at consumables for threat instead of spell damage on your gear slots. Your weapon will probably be a threat weapon due to the stats allocation anyway, so gemming threat is usually not as powerful in general.
The general rule here, though, is that if you lack uncrushability, gem to it, while if you have a solid basis of uncrushability, gem to Stamina.
Enchants
Helm: Glyph of the Defender, Heavy Knothide Armor Kit, Glyph of Power
Of these, Glyph of the Defender is the only worthwhile one. GoP is hardly worth considering for threat compared to the ~2% avoidance from Defender. Similarly, 10 stamina from the armor kit is substandard by comparison.
Shoulder: Inscription of Warding/Greater, Inscription of the Knight/Greater, Heavy Knothide Armor kit
Obviously, the greater inscriptions are better than the normal ones. Like the Glyph of the Defender, they're also better than the 10 stamina from Heavy Knothide Armor Kit. For that reason, work on your aldor/scryer rep.
Cloak: 120 Armor, 12 dodge rating, 12 agility
This is the hardest choice among enchants, in my opinion. Armor is the basis for effective health, which makes it a solid choice on any cloak. Agility is a DPS enchant, really, as established above in the gems section. Dodge is a good choice for avoidance, particularly if you're under the uncrushable benchmark. Personally, I agree with Ciderhelm's evaluation: enchant to the strength of the cloak. If the cloak is primarily for effective health, go armor. If it's for avoidance, go dodge. Devilshark should, for that reason, usually go dodge, while Slikk's can go either way, Gilded Thorium to armor, and so on.
Chest: Exceptional Health (150), Heavy Knothide Armor Kit, Exceptional Stats, Defense (2.4)
This one is an obvious choice for the 150 health enchant. You can't make the stamina armor kit compare. Noteworthy alternatives, though not nearly as good, are 6 to stats (60 health, 90 mana, .24 dodge) and 15 Resilience (crit reduction, but you should be getting that through defense unless it's a resistance set, where you need the health anyway). The 2.4 Defense enchant provides a significant boost in avoidance, giving approximately 1% towards uncrushability, .75% avoidance.
Bracers: 12 stamina (Fortitude), 12 defense (Major Defense), 15 spell damage (Spellpower)
12 defense is ~.6 total avoidance, while 12 stamina is 120 hitpoints before talents and kings. I prefer 12 stamina. 15 spell damage might be ok for a threat set, but is largely unnecessary as an enchant. As mentioned above, spell damage is best gained through your weapon and consumables, while mitigation and effective health are best achieved through your armor/enchants/gems. 12 defense is great if you're low on defense overall.
Gloves: Heavy Knothide Armor Kit, 20 spell damage (Major Spellpower), 15 agility (Superior Agility), 2% Threat (Threat), 240 Armor (Glove Reinforcements)
This is another very difficult piece to choose an enchant for. 15 agility = .6 dodge, which is a solid choice for the slot, while Heavy Knothide is 10 stamina. In the case of this slot, where the spell power enchant is so potent compared to avoidance stats, it's also a reasonable contender. Lastly, if you reach a certain (fairly high) spell power threshold, Threat does extremely well. Personally, I use Heavy Knothide. As with the cloaks, though, I recommend enchanting to the strength of the gear. Many tankadins swear by Major Spellpower and, truth be told, it is a very potent option. 240 Armor is powerful, but it sees poor returns until your HP is quite high. It's a great consideration for T5-T6 quality gear.
For a more detailed discussion of the glove slot, see the following thread: http://www.failsafedesign.com/maintankadin/viewtopic.php?t=7257
Legs: Clefthoof/Nethercleft, Mystic/Runic Spellthread
Nethercleft is the top effective health leg patch option. Honestly, nothing else holds a candle to it, in my opinion. A fair number of tankadins consider use of spellthread instead, however, for the increased threat. I'm not so anxious to pass up .48 dodge and 200 health unbuffed for 35 spell damage, however. The lower versions (Mystic/Clefthoof) are fine options for transitional gear to save cash, but when you do get a substantial piece that will last for a long time (badge leggings, for instance), Nethercleft is THE way to go.
Boots: Boar's Speed, Fortitude, Cat's Grace, Dexterity
These enchants are pretty much the only way to go as far as tanking is concerned. Personally, I value mobility, which makes Boar's Speed very attractive if you didn't already sacrifice 3 talents for Pursuit of Justice. Since PoJ doesn't stack with Boar's Speed, Fortitude is a superior option for those who spec for it. Mobility is something that Paladins struggle with, so having any little boost is quite significant, especially when it only costs 3 stamina. Cat's Grace and Dexterity are options for avoidance sets, providing .24 and .48 Dodge, respectively.
Rings: 12 spell power, 4 stats
While an obvious choice for any Warrior or Druid enchanter, a Paladin enchanter is sort of up in the air on this. As far as potency, 12 spell power is pretty strong. I prefer 4 stats as an option, as it gives more effective health. 4 stats is worth .16 dodge, 40 health, and 60 mana per ring, which I consider to be quite powerful. As before, enchant to the strengths of the gear, however. If you're using a spell power ring, use a spell power enchant every time.
Weapon: Mongoose, Major Agility, Major Spellpower, Battlemaster
This definitely falls into the category of enchanting to gear strength. I carry a mitigation weapon (Sun Eater) with Mongoose and a spell power weapon (Amani Punisher) with Major Spellpower. Major Agility is a more consistent option for tanking than Mongoose, giving a static .8% dodge, but seeing Mongoose proc an extra 4.8% dodge for 15 seconds every minute is quite rewarding. Essentially, it's like popping a trinket every minute. Spellpower is a stronger general enchant, giving a significant boost to threat, which is better for nearly 90% of the content you'll be doing. Sun Eater gets pulled out for certain extreme cases, such as Magtheridon's final add. Everything else gets the Gavel. Battlemaster is mentioned because, on certain encounters with AoE damage, the proc can generate a significant amount of threat. It is not a great enchant, by any means, however. Spellpower will generate more threat per second overall than the 1 proc per minute you'd expect out of Battlemaster.
Shield: 18 Block Value (Tough Shield), 18 stamina (Major Stamina), 15 Block Rating (Shield Block), Felsteel Shield Spike
This enchant is incredibly dependent on the shield you are enchanting and the gear level that you're at. 18 stamina is a great catch all, particularly for something like Crest of Sha'tar, where you can stack the stamina pretty high. Block Value is incredibly powerful on block value shields, like the Bulwark of the Amani Empire or the Arena PvP shields. Block Rating is powerful if you're not yet uncrushable, as it's nearly 2% total block, a dramatic increase for any starter tankadin. Finally, the Shield Spike is good for a damage shield, like the Petrified Lichen Guard, which you're unlikely to use in a raid setting. Generally, I prefer 18 Block Value because I'm a BV whore, but you should have a shield with at least 2 out of the 4 options on your person all the times. Block Value and Major Stamina are preferred, but not everyone has uncrushable as they're walking into Karazhan. Fortunately, all of these enchants are fairly inexpensive, save for the Shield Spike, which is more of a Warrior thing for OTing than anything. Lastly, yet again, enchant the shield to the strength of the shield. You will find fewer returns from Tough Shield on an Aldori Legacy Defender than you will on Shield of Impenetrable Darkness compared to stamina, while a Bulwark of the Amani Empire needs the extra Block Value from Tough Shield.
General Enchanting Principles
1. Always keep in mind the total effect that any given enchant has on your ability to tank content. Sometimes, the overall "lower quality" enchants are more viable for certain pieces of gear. You do your raid no good if you are dead or if you're not holding aggro against your DPS. For that reason, if you're having problems surviving, get survival based enchants, while if you're having problems generating threat, put on some threat enchants. Moreover, if Agility on your gloves pushes you over to uncrushable, get that over spellpower and, in rare cases, if defense to bracers makes you uncrittable where you otherwise wouldn't be, get it. The same principle applies to gems.
2. Enchanting to the strengths of your gear is a strong gearing principle. Tanks are not made best by a balanced set of gear in most cases, but a min/maxed set that specializes in something particular. This doesn't mean neglect stats, mind you, but it does mean that you can make a stronger set of Block Value gear by removing some dodge and vice versa.
3. Carry multiple sets of gear that specialize in different things. Different situations require different gear. Block Value is extremely powerful for trash compared to bosses, while effective health is very powerful for bosses with big, slow hits. Enchanting your gear to match the strength of the set is very important for making the set work properly for you.
4. Don't cheap out. As a tank, you have the most min/maxed position in the entire raid. That means that you should go get Nethercleft instead of Clefthide for your legs, spend the money on rare gems instead of cheapy Uncommons, and don't leave your gear without an enchant or a gem where you can have one.
Blah, blah, blah, I wrote this, I gave credit from things that I borrowed from others, etc. Discuss, argue, and maybe I'll update this based on the conversations if I'm not too busy getting hit in the face by bosses.
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