Honestly, it is not a one thing or two, but a huge list.
Starting from artifact. Me, for example, fo some reason thought we'd get a scythe, and since I'm a sucker for scythes as a weapon, that was a huge disappointment. Just imagine your shadow priest with a god fucking damn awesome SCYTHE with a blade made of smoking void... I just had a nerdgasm simply imagining it. Now, you can imagine how disappointed I was when I've seen - what - a fucking DAGGER. Which is cool lorewise, but visually it sucks, and transmog restrictions add to this A LOT. Like, if only I could transmog it to mace, it would instantly be 70% less bad... but... But.
Then, honestly, it just ran down the hill.
We had Oblivion, a real dps cd I've dreamed about since Cata. Removed. That was second huge hit on me wanting to play shadow in legion. Yes I know, SWV
kinda does the same - but it is a talent that steals our awesome execute, and I don't think I want to trade my execute for being able to burst when fight starts, not 20 secs after.
We had instant mind blast, the one and only thing that made shadow priest's gameplay in WoD actually awesome. Removed.
We had Spectral guise, feathers, reflective shield, cascade, halo, DS, Fear ward, Leap of faith, Void tendrils, DP, Desperate prayer/Shield, bonus armor on shadowform, PWF buff, resurrection and levitate removed (yes I know res and levi is back, so what? the list is HUGE even without that)
We have unimpressive spell visuals. In an expansion when every spec gets new visuals, and melees are even getting true abilities animations so they look like badass slasher heroes.
We finally had a defensive which was not silencing us for 100% of its uptime - now its gone, and we start with a dispersion nerfed for 30% for god knows how long.
We had a freaking awesome mastery... removed, replaced by generic stuff. Nope, I can't even call it generic, since I know no other class whose mastery buffs the damage of its three spells - and that's all. Imagine Fire mage getting mastery that increases damage of Fireball, Scorch and Dragon's breath. Or a fury warrior's mastery increasing damage of Bloodthirst and Wild strike. Cool, right? Nope, it sucks. Our mastery sucks as well. Its worse than a generic +% to spell school damage...
PvP talent tree got a lot of stuff I'd love to have in pve instead.
Our last talent tier, don't even get me started on it. StM the Void Entropy v2.0 in terms of usability, and mind spike which was stolen from us as well. And it is the last talent tier, the one which should give best stuff, the stuff which determines the way you play, which shapes you as a spec.
Serious lack of tools to properly respond to game encounters. I've already had my share of fun nuking one mob in a pack of five during all WoD, because that way I was dealing more damage than spamming mind sear the abomination of a skill. I don't want to continue lacking proper tools. I don't want to struggle beating fucking tank in challenge modes on damage for the third expansion in a row.
Dots were generating Insanity, like it should be. Than it was removed. And then it was placed as a bandaid fix in a tier set bonus. No commentaries on this one. The spec just got a fabled "rework" damnit, and we already get bandaid fixes... unbelieveable? nope, usual stuff. Then Blizzard started pushing AS, while still not fixed the main problem of apparitions AKA fucking travel time...
Shadowy insight as a talent... seriously, Blizzard? That talent was not good for ages, like since early Cata or something??
Insanity cost for utility spells, like mass dispel or silence. Even our fabled oh-so-strong heal costs insanity...
Dominate mind that is not working on mobs in Legion dungeons (reported by some alpha players).
SWD hits like wet noodles.
Nerfed VE to the point its not worth to use it as a raid cd. Nerfed SoI to the point its not worth it to refresh dots before going to VF.
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So, what do we have in the end? Who knows, maybe there is something huge coming in the next builds, like suddenly Blizzard are listening and fix all the problems shadow priests had for ages... maybe. But I seriously doubt it. I bet, all we have ahead is nerfs: -15% to swp damage, -20% to shadow mend, -15% to mind blast. And, when legion arises, chances are high we will be just where we always were: mediocre on single target, good on spread cleave, suck at everything else.
And that's not enough for me anymore. I want to play class that is at least exellent in one of possible game scenarios and good in one more. Which is, as it looks, almost any other damage dealing spec. Which is a shame, since I loved void form visuals as well as priest's new tier. But that's not enough
TL_DR Sorry for long, somewhat hysterical post. But I've spoken from my heart.