From doing some 2v2 arenas during the week in between Season 5 and Season 6, the build I'm favouring is
this Frost build (I hope that the link works, I'm at work at the moment on my lunch and the web security can be tight)
Most talents are obvious, although some are interchangable. As far as I have experienced and from other DK's advice, Scent of Blood has good potential if you're feeling starved of RP. From my point of view I haven't needed it (yet) as I'm playing around a style which allows 1 minute to build full RP.
Gone is the "easy" 0/21/50 (or 0/20/51) Shadowfrost spec, where we used to chase down healers and burst them from afar, burst them from close range. I've found that matches in 2v2 have become a lot more tactical from a DK/Healer point of view. Now we need to co-ordinate stuns and use offensive cooldowns wisely. From the last week's experience I've found that the following tactic gives the best results:
1. Build 130RP by any means possible, while converting 2 Blood Runes into Death Runes. You should be concentrating on the opposing DPS while having the healer set as your focus.
2. Death Grip the healer so that both opponents are in range of a Hungering Cold, then use HC.
3. Immediately go for the DPS while the healer is frozen by HC (disclaimer, when facing a Ret Paladin, it may be better to go for the healer, as they can cleanse their healer of the HC and you're back to a 2v2 situation)
4. Utilise 4 x FS, 3 x OB, Deathchill a FS, FS again and if Rime procs then Howling Blast.
5. If needed, Stragulate the healer when HC fades, or have your healer stun / fear the opposing healer to give you more time to get the killing blow.
All the while, your healer should also be DPSing the target to aid getting the kill. If and when a healer trinkets the HC, play defensively and repeat the sequence again 1 minute later.
A lot of discussion has centred around the Glyph choice and I believe, after a bit of testing, that FS-OB-HC are the way to go. The build is focused around a 10-15 second burst period every minute, and having a lower RP cost on FS, extra damage on OB and a free HC has given me the most kills during this burst sequence. I really feel that starting a burst on 130RP is a great bonus instead of having that RP depleted when using HC.
While testing I have used 2 different armor choices, one giving ~350 and the other sitting at 700+ resilience and I have had the same amount of success in each for kills, yet feel that the lower resilience hampers my survivability by quite a long way. FP and UA when focused helps a lot, but I just feel "safer" to go about my business with the added resilience. Weapon used was Ironsoul from Ulduar-10 with FC runeforged.
We'll see how arenas play out in the coming weeks, but I was impressed by the above spec. I lost quite a few matches due to simple mistakes coming from a lack of experience at the spec, but won quite a few outside of the CC-burst sequence due to some big FS and OB crits at crucial times.