Well I might as well add might 2 pence. I don't really have an opinion on Destruction, it's not a spec that appealed to me, so I won't be commenting on it. Demonology and Affliction on the other hand... The CM advised that we discuss 'by spell' basically what does and doesn't 'feel good' about them, so I'll try to keep to that.
Affliction
All three DoTs feel exactly the same. Three timers, all instant cast, no special graphic or feedback particularly that any of them feel you've done something.
They all each feel individually very weak; in fact using the Soul Effigy talent requires you to cast each twice to get more power out of them. This is layered then with either UA as a 'maintenancy' debuff or Mana Tap as a maintenance buff to then bring them up to full power; and further maintenance buff in Reap Souls as a final flourish. It's probably the most work we've ever had to do just to make our base spells work.
3-6 DoTs, and 2 maintenance buffs. That's a lot of timers on my UI.
I'll give special attention to Reap Souls here. It's an artefact trait, and it's the type of spell that has been stripped away from other classes. I look at what other classes and specs get, and they get some big nukes; things that impact play to use in conjunction and synergises with other abilities. We get a maintenance buff. It's dreadful, and smacks of the last ditch change because the previous iteration was somehow worse. It adds nothing to the spec or gameplay, but another timer and barely meaningful resource.
Life Tap. It is iconic, or at least it used to be, but now it's another zero impact instant cast spell you press when a bar moves too far toward empty.
Drain Life as a filler is fine, it feels no different to it ever did, or Drain Soul or Malefic Grasp did; but the visual is a step backwards from those two and that makes it feel as weak as it perhaps is.
Mana Tap. I'm not one who's against maintenance buffs by default; they can add something if they have a nice visual impact, if it has a clear benefit across all targets - as was with Soulburn Haunt - or has some other secondary benefit, like an opportunity to move within a heavy cast time spec. This does none of those. Someone dropped the ball when they could have given this the Haunting Spirits visual and instead went for some barely visible 'twinkle' which is absurd and only makes it harder to track. And we're already casting infinity instant-cast soulless spells, which hit for nothing, so while the benefit is there, you barely feel it.
Soul Effigy. There's a special place in hell for this. Not only is it an awful 'dot throttle', doubling the amount of work required to achieve "normal", it's also awful in the UI consuming the focus target, it's placed behind you at feet, meaning you have to place it (ideally in melee for AoE synergy), and it's targetable by pets, making 'assist' entirely unusable if you have this talent. Even for those who thought this was a good idea because 'if you like to multidot', it's still terribly implemented for that goal.
Phantom Singularity. Literally the only spell in our arsenal that feels like it is doing something. It's a great button to press, but that comes around far too infrequently. Adds in fights, and in dungeons aren't on 1 minute timers, so it's just not always going to be there when you need it. It's self limiting in that respect, which makes it questionable. It's definitely the nicest spell on the row, and without a massive cast time as Cataclysm had, it feels much more user friendly when it is ready - I'd honestly take a 1.5s cast on it to cast it more often.
Soul Conduit is not an exciting passive option, it feels weak because it's barely noticable and doesn't really have an obvious synergy with other spells like Effigy has.
Soul Shards. Can we please please have them back on our character? Please? It looks cool, and it's so much easier to track than modding our interface even further. Please.
All in all, the spec feels like it has an awful lot of buttons, and awful lot going on, but nothing really individually that actually does anything. There's a lot of duplication of purpose and effect that needn't be there.
Demonology
I'm sure there's good reason you didn't put a combo-point system on a caster before, but as with so many other issues with the spec that were given as to why not to make a summoner spec; they just went out the window. Plainly Rogues with their combo points are not a popular class, often described as slow and repetitive; which Demo is in spades.
Demonic Empowerment. I've not seen anyone, anyone say anything good about this spell. It's just a soulless, empty cast time you add on to each and every summon. It's tedious. It has no feedback at all. It makes your weak minions slightly less weak, but not strong enough to notice individually.
It also takes a very long time to build up that "army" of Imps and Felstalkers, given they're all on cast times, with a cast time on top of them, and require building resources from a cast time spell. I think 14 seconds is the time given for it to reach "normal" levels. In a dungeon, you'll be through at least one trash pack in that time.
Doom. It's always been bad for dungeons, so doesn't really fit the 'new game' that includes Mythic+. In many cases, the only reason you'd apply it, is to get your Darkglare to attack the thing.
Darkglare. Why change this from the very popular brand new Inquisitor to this tedious 2006 eyeball by the way? Just an aside. The fact it needs Doom to prompt it to attack things is bad. It was bad for the Doomguard in Cataclysm, it's still bad now. Nothing really changed to make it not bad, so why you think this is no longer bad, I don't know. Make it just attack stuff we attack, like the new assist is supposed to do.
Implosion. Very obviously this will reset your ridiculous ramp up to deliver some burst. I think this is a problem, as the adds themselves may not be a major priority, and a few weeks of progress will render this spell redundant when every other spec in the game can cleave things down without having to reset their boss throughput. Spells shouldn't become redundant like this.
Guardian pets in general are horrible when it comes to switching targets. That's a problem. Having a massive chunk of your damage continue to attack a target that for very good reason you might want to actually stop on to prioritise something else, is a big part of encounter design.
This spec clearly wasn't designed with encounter design or dungeons in mind.