In most of my analysis you will see disregard 'threat' stats like Strength, Hit
and Expertise. This is done with the belief that threat is a minor issue right
now. Gear decisions should be based on Survival stats alone. You'll get 'enough'
threat stats that come along for the ride.
From everything I've read about DKs, there's an interesting contrast. In most cases, DKs are fine for survival, but have some issues with threat generation. According to the EJ thread on DK tanking, most DK tanks once they get some gear on them are fine for taking damage (large health pools and armor levels for their gear quality make them easy tanks to heal, plus a lot of low cooldown "Oh Shit!" buttons).
Since I already am well over 540 Defense (was there before the patch, now I'm even more firmly above thanks to the Sigil of the Unfaltering Knight and the Rune of the Stoneskin Gargoyle), I can consider other gear and gems than stacking defense. There are a number of options available to me.
As a general priniciple I'm going to completely ignore Red, Orange and Yellow gems since they don't have Stamina on them. Stamina is a "Blue" gem stat, so only Green, Blue and Purple gems have it on them.
Enduring: +Defense Rating and Stamina: While I'm already at the Defense minimum, these gems are still valid since they will allow for either freeing up Defense elsewhere if you would prefer gear that has little or no defense on them (Some good rings are heavy with Dodge Rating, for example). Also a very good option after you hit the cap for spell hit.
Vivid: +Hit Rating and Stamina: Since a Death Knight relies on both magical and physical damage, hit is a very important stat. By increasing hit, you'll have less misses on all of your attacks. Also, a miss means a thrown off rotation, which can be really bad for a Death Knight since they should be active almost 100% of the time.
Solid: +Stamina: Obviously this is a high value gem, since tons of stamina will help out against all situations.
Guardian's: +Expertise Rating and Stamina: Much like the Vivid gem, the Expertise lowers the chance of a Parry or a Dodge, which means that there will be less thrown off rotations. Plus Expertise will remove our minimal chance to get hit by parry-gib, increasing our survivability. It's wonderful to have double-duty stats.
Soverign: +Strength and Stamina: With threat being an issue, having more Strength helps to fix this issue, plus Death Knights have an ability called Forceful Deflection which gives 1 Parry Rating for every 4 Strength a DK gets, making Strenght another double-duty stat.
Regal: +Dodge Rating and Stamina: If you find yourself dying, but not having issues with threat, this is your best option for additional survivability.
Defender's: +Parry Rating and Stamina: The most undesireable of the various purple gems available to a DK. First off, due to just having a lot of Strength (my DK in a mix of blues and purples has around 800 STR) they will have a lot of Parry Rating. Since defensive rating stats (Block, Parry and Dodge) all suffer from Diminishing Returns now (implented, I believe, because of the Rogue who tanked Gruul and other bosses), so since a DK tank will already have a lot of Parry from Strength, getting more will do less than getting a Regal gem in general.
There is the option of choosing a AGI and STA gem, but that's a really bad choice in my opinion, since it gives so little crit, armor and dodge that it's almost painful, plus there's zero crit on tanking gear. Since crit is a stat that you get more for having more, since you're at an absolute minimum while in tanking gear, getting a little more is barely worth it.
Anyway, for now I'm gearing myself up with Vivids, Solids and Guardians'. I'm almost at the dodge cap for Expertise (have 21 expertise right now and need 23 or 24). I'm not sure whether I should concern myself with getting the additional 10% to completely remove parry chance. That's a lot of Expertise rating. Also working on getting my Hit capped for spells since I use Howling Blast and Icy Touch a lot as a Frost DK.