I've done posts about Blood and Unholy first because, frankly, they're the two DK specs that are getting the biggest changes. Frost Death Knights have most of the same abilities and procs, but there are some significant changes coming, primarily as Blizzard consolidates Frost's two-handed weapon and dual-wielding playstyles.
All Death Knights will now have six straight up "Runes," no more Blood, Frost, Unholy, or Death Runes - though one could argue that all runes are Death Runes now. While they are no longer paired up for the purpose or rune regeneration, you'll still only be able to regenerate three runes at a time. Runes still generate 10 Runic Power when used, so an ability that costs two would give you 20, etc.
Frost DKs still have Runic Empowerment, but it's much simpler to use thanks to the aforementioned change to the nature of runes. You'll now have a percentage chance per Runic Power spent to regenerate a fully-depleted rune (which you should have three of at any given time.) I don't know for certain how Sindragosa's Breath works with this (it's still a talent,) but I'd assume that each time it ticks and spends RP, it'll do another check.)
And of course, now that all runes are interchangeable, there's no more gaming Runic Empowerment by leaving one of your Unholy Runes unused to ensure that it's a Death or Frost Rune that gets reactivated. You can just spam your Frost Strikes as much as you like.
Frost has officially become the dual-wielding spec for Death Knights. Your abilities actually won't even work with a two-handed weapon, so you're going to have to stick with a pair of weapons. I haven't tried starting a new DK character at 55, but I'm curious to see if they've updated the quest rewards in the Death Knight starting experience.
Obliterate and Killing Machine:
Killing Machine now only affects Obliterate. This might push the spec to working much more like the old two-handed rotation, which focused on big Obliterate crits.
On the other hand, this might make the proc feel kind of like a random bonus rather than something that changes the rotation. I could imagine that with a lot of mastery, you might go back to the Howling Blast spam version of the spec, but interrupting that for Obliterate when you get a Killing Machine proc. I'll wait to see what the theorycrafters say, but I've been having some decent success on the PTR focusing more on Obliterate.
Frost Death Knights no longer have Death and Decay. Instead, they'll use Remorseless Winter, which similarly lasts 10 seconds and has a 30-second cooldown and costs one rune. However, Remorseless Winter of course does Frost damage and it works as a buff that you place on yourself, damaging enemies that stand near you. I imagine this could work as part of the single-target rotation, though again I'll have to see if it's worth wasting half an Obliterate.
That's really the big changes. For the most part, this Frost spec is the one you've known. Frost Strike is still your main RP-dump, and there are some cool new animations (like most melee classes.) I suppose my only regret is that with Obliterate counting its two weapon strikes separately, you won't see the enormous numbers on screen that you did when critting with it as two-handed Frost.
If you're on the fence about going Frost on your DK, remember that their artifact weapon is a pair of swords forged from the shards of Frostmourne. This is the closest you'll get to actually wielding the most iconic weapon in Warcraft, so... could be cool. (Believe it or not, that pun was not actually intended.)