While I've been more of a Frost Death Knight since the beginning of Mists, I'm going to focus more on Blood here, because Frost is actually probably the least-changed DK spec - essentially, the 2H rotation of Warlords will be the baseline one, and you can talent to make it more like the current Dual-Wield one.
Blood is getting more changes, particularly in that it's becoming refocused on melee strikes, though there are certainly some other big re-jiggerings of the spec. It's still very much based on self-healing and absorption, but the manner in which you achieve these things is changing.
Before we get to all of that, though, let's touch on the change to Runes. Gone are individualized Blood, Frost, or Unholy Runes. All Runes are now effectively Death Runes, though we just call them Runes now. They each have a kind of icy-blue skull as the symbol. You'll still only be able to regenerate three at a time, but I don't think they're paired up the way they are now. As usual, each Rune spent generates 10 Runic Power unless otherwise noted.
And here's the usual caveat - this is Alpha, so things can and will change.
Blood Plague is now your only disease (each spec gets a single disease.) It deals shadow damage and heals you for as much as it damages, making it potentially very effective when spread between many targets.
Blood Strike returns. costing 1 Rune and dealing physical damage, applying Blood Plague and also generating a bonus 10 Runic Power (so a total of 20.)
Death Strike has gotten some big changes. It now costs 40 Runic Power instead of Runes, becoming your main RP-spender. It's back to the Mists/Cata version in which it heals you based on the damage you've taken in the last 6 seconds (which is needed now that Resolve is gone,) with a minimum heal of 7% of your max health, but also, if you're below 35% health, it will consume all Runic Power you have to enhance the heal by an amount proportional to the extra RP consumed. Given that it now costs Runic Power, the availability of the strike will be a little more predictable.
Marrowrend is a new ability that costs 2 Runes, dealing physical damage and giving you three charges of Bone Shield. I suspect this will be the main thing you spend Runes on once Blood Plague is up.
Death's Caress is a kind of ranged Blood Strike, costing 1 Rune, dealing Shadow damage and applying Blood Plague. It's less damage than Blood Strike, though, so when in melee you'll want to use that.
Blood Boil remains unchanged, but of course you'll only use it in multi-target situations now that we have Marrowrend and other stuff to spend Runes on. It still spreads and refreshes diseases, so basically any time you have more than one target, you can probably just sub this in for Blood Strike once the disease is applied.
Death and Decay works pretty much the same, but standing in it causes Blood Strike to hit all nearby enemies (much like the HotR/Consecration combo for Protection Paladins.)
Crimson Scourge now causes autoattacks on targets afflicted with Blood Plague to have a 25% chance to reset the cooldown on Death and Decay, which of course will be easier to cast since all Runes are universal now.
Dancing Rune Weapon remains, but it now buffs your parry by a full 50%.
Anti-Magic Shell remains, I believe largely unchanged.
Vampiric Blood has a much longer, 3-minute cooldown now, but it increases your health and healing received by a full 100% (which is arguably the same as cutting the damage you take by 50%.) But there's no more Icebound Fortitude (at least for Blood.)
Putting it all together, and this is of course before talents and artifact traits, the rotation seems like a lot of similar elements to what exist now, but in different places.
Runic Power Spenders/Active Mitigation:
Here's the easy part:
1. Blood Strike.
1. Blood Strike to maintain Blood Plague.
2. Death and Decay.
3. Marrowrend if you have 7 or fewer Bone Shield charges.
4. Blood Strike (which you'll probably want to use instead of Marrowrend if you can, given the bonus RP from it.)
Actually, that's not too bad. You'll obviously be able to use Death's Caress to get your disease ticking on distant enemies, giving you a bit of threat and healing if you can't or shouldn't Death Grip them to you.
In AoE, you'll use the same abilities, but probably swap out Blood Strike in favor of Blood Boil once the diseases are ticking.