Demonology is now focused on summoning demons rather than becoming one. There are a couple of returning spells, but the binary nature of the 5.0-6.2 Demo Lock looks like it's going away.
Also, all Warlocks will once again go back to using Soul Shards (though thankfully Cataclysm-style, as a class resource rather than a physical item you have to keep a special bag for, like it was before Cataclysm.)
So let's look at abilities:
Shadow Bolt remains, and I believe we'll continue being the only Warlocks who continue to use it past level 10. You'll use this to generate a Soul Shard, which gets spent on stuff later on the list.
Corruption remains as something you'll need to keep up on the target.
Call Dreadstalkers is the exciting new thing. It costs Soul Shards (I don't know how much, since it currently lists it as "0 Soul Shards," which has to be a placeholder.) You'll summon two Dreadstalkers (those sleek new Felhounds we saw in Tanaan) to attack the target for 12 seconds, with a 15-second cooldown. So you'll be able to maintain these guys most of the time, but they will have some downtime (plus the two-second cast time.)
Hand of Gul'dan now has a cast time (again) and I believe consumes all your Soul Shards, dealing more damage per shard and also summoning a Wild Imp with each shard consumed (who last 12 seconds.)
Demonwrath appears to be your main AoE spell. It's a bit like Hellfire, but no longer damages you, and it instead is centered around each of your summoned demons. The idea will clearly be to have as many demons up as possible and then go nuts with this. Each time it deals damage, it has a chance to grant you a soul shard, which should make it easier to maintain your Imps (and maybe even get a nice feedback loop with more and more imps getting summoned thanks to the soul shards.) Like Hellfire, which it appears to be replacing, you'll be able to move while channeling this.
Demonic Empowerment will, I suspect, be the key to playing the spec well. It has three charges, a 12-second recharge and a 1.5 second cast. It increases the haste and health of your three strongest demons by 50% for 12 seconds. I assume the ideal situation here will be to use it to buff your Felguard and your Dreadstalkers, or to buff your
Soul Link is a baseline passive, and I think it works more or less the same as it does now.
Doom remains (though it should be less of a juggling act to maintain it with Corruption.) It works a little bit more like it used to, only dealing damage when it ends, but it also only lasts 20 seconds now. It also generates a Soul Shard when it goes off.
Mastery: Master Demonologist now simply buffs Demonic Empowerment, further cementing that as the core of pulling off the rotation correctly.
So before we get into talents or the abilities of the artifact weapon (and I still have no idea what the Skull of the Man'ari will count as,) we can figure out a basic rotation/priority.
1. Maintain Corruption and Doom
2. Use Demonic Empowerment when you know that you have some powerful demons who are going to be up for a while - basically try to cast it right after your Dreadstalkers come out.
3. Use Hand of Gul'dan when you've capped your soul shards and Dreadstalkers are on cooldown.
And then for AoE, it's going to be some kind of mix of spending shards on Hand of Gul'dan (you might skip the dreadstalkers just to have a larger number of minions) and generating them as well as doing lots of damage with Demonwrath.
I do suspect that some of the numbers, particularly cooldowns, cast times and durations of demons, are going to change a lot here. It does also move the spec away from managing DoTs (though you'll still probably want to light everything up with Corruption and Doom) and toward managing minions, which seems far more appropriate to the spec.