Frost Death Knights are going to be dual-wielders in Legion, and probably moving forward. My Death Knight, who is kind of my Prime Alt or Vice Main - the number 2 character I play - has been a two-handed Frost DK for two expansions now. He was originally Blood DPS, but then they got rid of that spec - that version of the spec, I should say - and so I switched to Frost, as it seemed like the closest analogue. In time, I actually grew to really like the rhythm of Frost - timing things around Killing Machine, for example.
Frost has this odd mechanic that is meant to distinguish between dual-wielders and those using two-handed weapons. This has actually always been a problem, and in retrospect, it's probably a decision that Blizzard regrets. Notably, Monks barely see a difference between dual-wielding or using two-handed weapons, and Fury still plays essentially the same way with its two weapon style choices. Only Frost has a totally different priority/rotation based on which kind of weapon you prefer to wield. I believe it Cataclysm it was flipped from its current form, but regarding the current model, wielding a two-handed weapon will buff the damage of Obliterate, a heavy-hitting all-physical melee strike that costs one Frost and one Unholy rune. Dual-wielding two weapons instead buffs the damage of Frost Strike, an all-Frost damage strike that costs Runic Power.
One of the most important passives for Frost is Killing Machine. It gives your (main hand) auto attacks a chance to make your next Obliterate or Frost Strike guaranteed to get a critical strike.
The idea, then, is that those with one-handed weapons will try to time it so that they use Frost Strike when they get their Killing Machine proc, whereas those using two-handed weapons will try to use Obliterate.
On the surface, this doesn't seem like much of a big deal. But in practice, it makes a massive difference that essentially ensures that the two styles can't really be balanced, and generally means that two-hander players will inevitably fall behind.
The missing piece is Frost's mastery, Frozen Heart, which increases the frost damage they do.
Because DW Frost's biggest attack is a pure-Frost damage ability, they are incentivized to stack a bunch of mastery with their gear. Mastery then buffs Howling Blast and Frost Fever (which is applied by the former,) making DW's splash/aoe damage very good.
2H Frost, however, doesn't really like Mastery so much because their biggest strike is the pure-physical Obliterate. But they're still weighed down by having the same Mastery - it's just that for one style, the benefit of Mastery is quite large, while for the other it's a much more minor effect.
So, you may then ask: why keep 2H Frost around? Clearly it's always going to scale worse than DW, and indeed these days the more serious raiders will overwhelmingly go DW, it just puts out better numbers.
We can answer that question two ways. The first is: We won't. Blizzard is getting rid of the sub-spec and making Frost a dedicated dual-wielding spec.
But to answer, rather than dismiss the question, here's why I'll be sad to see the subspec go. I won't give this as a reason that it should stay, because I don't think there's any practical chance that it will.
The Fantasy of Frostmourne:
It's true that there will still be Unholy for those who like fighting with a big old two-handed weapon (and of course, Blood tanks will continue to use one,) but Frost is kind of about purity. Unholy's sources of damage are far more diverse. You have Scourge Strike, which is split into Shadow and Physical portions, plus your ghoul, who needs to be managed. 2H really feels powerful to play - you hit things, and you hit them really, really hard with your Obliterates. Unholy plays more of the necromancer/undead commander role, whereas Frost feels more like a Champion of the Scourge - Unholy summons the monsters, while Frost is the monster. Dual-wield breaks up the damage of its strikes between two weapons, and they don't get to carry a big, looming sword, but must make their weapons kind of share the spotlight with each other (though admittedly, having a pair forged from Frostmourne's shards might make me overlook this.)
The Spamminess of Dual-Wielding:
It's totally possible that this has been fixed and I just haven't seen it yet, but toward the end of Mists of Pandaria, Dual-Wield Frost didn't really have a use for Obliterate anymore. Because they were stacking up Mastery to buff Frost Strike and Howling Blast, it got to a point where being able to hit Howling Blast twice more became a better option than hitting Obliterate at all. So, what should have been "Obliterate when there's no Killing Machine, Frost Strike when there is or you're capping your Runic Power, and then Howling Blast when it procs," became "Spam Howling Blast, Spam Frost Strike." It was less interesting, and Killing Machine lost its interactivity.
Diagnosing the Problem:
I don't really think that weapon choice is totally to blame here.
The problems I see are with Frozen Heart and Killing Machine, or looking at it from another angle, the passives formerly known as Threat of Thassarian and Might of the Frozen Wastes.
Killing Machine itself has the problem that it devalues Critical Strike as a stat. But that's not too bad - other specs and classes have to deal with stuff like that. But the proc loses all of its meaning if the player is able to drop the "lesser" attack from their rotation.
Frozen Heart is not a terribly interesting Mastery - but that's not always such a bad thing. The real problem with it is how it benefits one subspec so phenomenally more than the other.
From a pure power perspective, Frost is built to dual-wield (having two weapons even allows you to use Rune of Razorice, which further buffs Frost damage.) Yet from the point of view of designing the spec to be fun and interesting to play, two-handers have the edge, as they don't risk falling into two-button mode, no matter how well geared they get.
Prescribing a Solution:
I don't think it'll be enough to just make dual-wielders (which will be everyone) favor Obliterate. But it would be better, as you could balance the spec around its physical/magic ratio being set, rather than changing based on a weapon choice that will no longer be part of the game.
Still, I think that a greater revamp of the spec might be the best way to improve things. I don't have terribly specific ideas about such a revamp, but I definitely think that Obliterate needs to be a compelling part of the rotation, regardless of how much Mastery you stack up. That might mean designing a new mastery, or changing the way that Obliterate works.
Clearly, the spec should still be based on melee strikes. Blood currently suffers from the fact that Death Strike is the only melee strike they have left (after Heart Strike and Rune Strike disappeared.) Death Knights are all about their weapons, which is why it's weird that Blood is so focused on casting spells. Frost is ok here - Howling Blast is the right kind of quick-burst spell that allows you to get right back to your melee attacks.
I'd also argue that, to be frank, Frost needs to be the "big numbers" Death Knight spec. I know that it's ultimately not a big deal - Affliction Warlocks can do fantastic DPS even if the individual numbers on the screen are small - but with Unholy going more the "death of a thousand cuts" route, Frost really ought to be able to get those nice big crits (or whatevers, if they wind up changing Killing Machine.) Honestly this might be as simple as just changing the UI so that two-weapon strikes show the sum total damage of the strike, rather than showing two numbers pop up, but I also think there needs to be this sense that while Unholy is rattling on a snare drum, Frost is banging a gong.
We're still not even in alpha for Legion (at least not in any public capacity,) and Blizzard hasn't done any in-depth class reviews for the expansion (something they might be saving for Blizzcon.) But I hope that we'll soon see what they have in store for Frost Death Knights. I've grown quite fond of this playstyle over the last few years, and I hope that I'll have something familiar or at least similar moving forward.