Comparatively, we are probably one of the worst casters at the current point in time, when it comes to out single-target DPS. Our multi-dotting capability is the only thing that make us competitive on encounters with multiple kill targets, and still then, there are other classes that do this better. There are a couple things that I believe are holding us back. Our complete lack of mobile DPS, which the majority of bosses seem to favor this tier, and the most popular Shadow Priest complaint since the ability had been implemented in game, Shadowy Apparitions. As most Shadow Priests know, Halo is our highest DPS-per-global for single target if you're using the spell correctly and and are standing at 25 yards. This being said, most SPriests are standing at max range to maximize the effectiveness of this spell. While doing so, we're also minimizing the effectiveness of Shadowy Apparitions. I believe the current workings of Apparitions right now is: Four Apparitions can be walking towards a target at a given time, and you can also 'queue' a max of 10, meaning as soon as one damages a target, another will spawn and beginning moving towards its target. PLEASE CORRECT ME IF IM WRONG. So to maximize Shadowy Apparitions would require us to stand as close to the kill target as possible to minimize travel time of Apparitions. The two spells counter each other completely.
Our mobile DPS was the other concern I had talked about in the above paragraph. As of right now, we have two options that enable us to DPS on-the-move. Most of us are spamming Shadow Word: Pain while moving as its initial damage is instant and we can spam it as our mana will allow it. The other option is to drop Shadow Word: Insanity and to pick up From Darkness, Comes Light. I have not done any theorycrafting on the two talents, but I'm 99.9% sure that Insanity always beats out FDCL. So our only option is to cast SW:P as we move. I'm not too fond of this, I would honestly like to see FDCL become a passive Shadow Priest ability and to have another talent created in its place.
TL;DR - I would like to see our spells mesh a little better and make a little more sense so we can maximize all of our spells and not have to choose between one and the other. I would also like to see a gain in our mobile DPS.
Thanks for taking the time to read and comment. :D
Edited: Updated thoughts on Shadow Priests's abilities and ways to improve the class overall.