With the introduction of Demon Hunters, Demonology had to lose its Metamorphosis focus to give that to the new class (that it kind of stole Meta from in the first place, to be fair.)
So what does the spec get in return? Demons. Lots of Demons.
Granted, Demo has had swarms of imps accompanying it every since the Mists of Pandaria revamp. But you'll be paying a bit more attention to your demons and summoning a larger variety in Legion.
So let's break it down:
Yes, all Warlocks are back to using Soul Shards, but rather than some weird utility mechanic, these are now a pretty standard secondary resource. You'll still have to Life Tap occasionally to regenerate mana, but the main thing you'll be watching is your Soul Shard meter. You get a max of 5.
The absolute most basic Warlock ability, Shadow Bolt is your primary Soul Shard generator. You'll be casting it a lot, but it's ultimately the spec's filler spell. It generates one Soul Shard per cast.
This is one of the two spells (before we talk about talents) that you'll be spending your Soul Shards on. This will summon a pair of Dreadstalkers (the leaner Fel Hunter dogs introduced in Tanaan Jungle) who will attack the target. You do need a target to summon them, but I think they will attack other targets if yours dies. You can talent this to have them come in with Wild Imps riding on their backs, which is both hilarious and awesome. These guys last for 12 seconds and the spell has a 15 second cooldown, so you'll be able to keep them up pretty frequently, but not constantly. There is a talent that causes this to sometimes proc to not cost any Soul Shards, which should make it easier to complement these guys with Wild Imps from hand of Gul'dan (see below.)
Hand of Gul'dan:
I suspect that this is going to be the spell you cast when you have Soul Shards to burn and Dreadstalkers are on cooldown. It does a bunch of Shadowflame damage to the target and nearby enemies and then summons a Wild Imp for each Soul Shard consumed, up to a max of 4. I believe the damage is independent of the Soul Shards, so in snap-AoE situations it might not be a terrible idea to spam this. However, if the enemies are likely to survive for any serious amount of time, I'd try to get those Imps up there for reasons I'll get to. Hand of Gul'dan can also be talented to apply Doom, which should smooth over your rotation a little bit.
Doom is your only DoT, though it's only kind of a DoT, since it only ticks once at the end of its duration. If re-applied, the "first" Doom will still do its damage independently of the second, so you shouldn't have to let it fall off. It also generates a Soul Shard when it goes off.
This is a spell that will require some skill to use properly, I think. It simply buffs your Demons' haste and health by 50% for 12 seconds. I'm sure you'll want to cast this immediately after summoning Dreadstalkers, and any temporary demons like your Doomguard/Infernal or the talented Darkglare. You obviously want to have this buff up on your demons as much as possible, but you also can't just cast it after every spell, because it doesn't do any damage on its own.
Similar to Hellfire (which this has replaced,) Demonwrath causes all of your summoned demons to radiate Shadow damage while you channel it. You can channel it while moving, and it has a chance to generate a Soul Shard each time it deal damage. Obviously, following a Hand of Gul'dan with one of these should be a pretty effective use of AoE, and of course the Soul Shard generation (which should be insane in large groups - with all your demons dealing damage to all those enemies) feeds into more Hands of Gul'dan.
So basically, it's a very different-feeling spec, but the mechanics are both flavorful and tricky, which should make it pretty fun.