Shadow Priest DPS

[Vengeance]

Thought I'd do a small guide for DPSing as a shadow priest, considering the growing number of priests in the guild recently.
This isn't going to be incredibly in-depth, but I'll try to convey all the tips and tricks I know of for maximizing your DPS. Keep in mind that is all largely from my perspective, based on what we've raided on normal or what I've done in LFR. Feel free to PM me here or in game with any questions you might have.

Stat Priority
Intellect > Spirit/Hit > Haste until 8085 rating > Crit > Mastery
Get your hit up to 15% using a combination of spirit and hit rating, and then try your best to get to 8085 haste rating. This will add two additional ticks to your [SPELL]589[/SPELL] and [SPELL]2944[/SPELL]. Do not, however, sacrifice your hit cap or more than 250 intellect to do this.
For gemming, follow these basics:
Below ~505 ilvl: [item]Brilliant Primordial Ruby[/item][item]Purified Imperial Amethyst[/item][item]Reckless Vermilion Onyx[/item]
Between 505 - 515 ilvl: [item]Reckless Vermilion Onyx[/item][item]Energized Wild Jade[/item][item]76699[/item]
Above ~515 ilvl: All [item]76699[/item] except to get Haste and Intellect socket bonuses

Talents
Shadow's talents are pretty representative of what Blizzard was attempting to do in MOP with talents. Many fights will require you to pick options different from your typical spec. I'll only be covering the tiers that directly deal with DPS:

Tier 3
[spell]139139[/spell] (PW:I) is generally the best pick of this tier. It boosts the damage of your [spell]15407[/spell] whenever [spell]2944[/spell] is on your target.
[spell]109186[/spell] (FDCL) gives a 15% chance on each [spell]34914[/spell] tick to grant you a charge (can store up to two) of instant, +50% damage [spell]73510[/spell]. It's superior to PW:I when there's more than one target, and it's a great alternative if you need some burst during the fight though it's somewhat random and proc-based.
[spell]123040[/spell] is also an option for single target fights, however it's barely better than Shadowfiend and somewhat lackluster when compared to its tier partners.

Tier 5
This tier is where you'll probably be switching around talents the most.
[spell]109142[/spell] gives you a 10 second buff that increases all damage and healing by 15% whenever you damage something under 20% health. This is your best choice when there's a lot of adds that you know will constantly be dying. On fights like Spirit Kings or Elegon it's fairly simple to maintain about 50% uptime on the buff, essentially allowing you to boost your damage by 15% for as long as you can keep up the buff. Keep in mind that when you get the buff, you'll have to refresh your dots. The better you are about reacting to adds and learning how to abuse them for this purpose, the more you'll benefit from it.
[spell]10060[/spell] grants you 5% more damage and 20% extra casting speed, and it stacks with Heroism. You'll have to refresh your DOTs here, too, to get the benefit. This is the best talent for most encounters.
[spell]109175[/spell] grants your [spell]589[/spell] ticks a 5% chance to give you a free, instant Mind Blast, generating extra Shadow Orbs and allowing you to pump out more [spell]2944[/spell]s. This will be your best choice for when there's more than one target on a consistent basis.

Tier 6
[spell]120517[/spell], or as Koth put it, the exploding Death Star spell, is the optimal choice when you can manage to use it within 20-25 yards each time, and everything is more or less spread out.
[spell]121135[/spell] is better when things are somewhat more closely packed and the spell is able to bounce to its full extent.
[spell]110744[/spell] is the New Jersey of this tier, and should be merely laughed at.

Glyphs
There are no mandatory glyphs for shadow, however a few do shine above the rest. [item]Glyph of Mind Flay[/item]  greatly helps in moving from fires faster, as the speed buff provided by the glyph will be up most of the time. [item]Glyph of Dark Binding[/item] is also great, as it allows for some renews or PoMs when you have nothing to damage, or your healers need some assistance. Lastly, [item]Glyph of Mind Spike[/item] is useful for when you're hard casting [spell]73510[/spell] (for example, Elegon's power sparks).

Rotation
Here's the core part of playing shadow. Before I get into the actual rotation, let me just advise that, as the case is with any damage class/spec, maximizing your GCDs and reacting quickly and intelligently to certain mechanics is the real key to great DPS. If you give it your all, it'll show.
One important thing to note for shadow, as it's a DOT-based spec, is that DOTs "snapshot" your stats when they're cast on a target. What this means is that if you cast [SPELL]589[/SPELL] with 25% haste, it will function with that 25% haste. Should you get a buff that increases your haste by, say, 20%, [SPELL]589[/SPELL] will continue to tick without that addition. With that in mind, you should refresh your dots whenever you get some sort of significant buff, e.g. Heroism, trinket intellect or large haste boost, synapse springs. Critical and mastery buffs, unless they're massive, should not be considered when you're thinking of refreshing a buff. For shadow priests, this concept applies to [SPELL]589[/SPELL], [spell]34914[/spell], [spell]2944[/spell], [spell]15407[/spell] , [spell]48045[/spell], and yes, even Shadowfiend/Mindbender.

Basic Priorities:
#1 - [SPELL]2944[/SPELL] with Shadow Orbs x3
#2 - [SPELL]8092[/SPELL] on CD, especially with [spell]109175[/spell] procs
#3 - [SPELL]32379[/SPELL] x2 below 20%
#4 – [spell]15407[/spell] :Insanity while [SPELL]2944[/SPELL]x3 is up
#5 – Maintain [SPELL]589[/SPELL] / [SPELL]34914[/SPELL]
#6 - [spell]120517[/spell] or [spell]121135[/spell]

If you don't have [SPELL]2944[/SPELL] up, or just didn't pick Insanity, [spell]15407[/spell]  is merely used as a filler spell and should always be interrupted to cast anything else. While on the move, spam [SPELL]589[/SPELL] for the instant damage, and use any procs you get (FDCL, DI) right away.
When you've got a number of targets, like Stone Guards, you'll want to maintain [SPELL]589[/SPELL] and [SPELL]34914[/SPELL] on all targets and use your procs appropriately. With FCDL and DI, you'll probably be getting a steady stream of Mind Blasts/Spikes, allowing you to basically ignore [spell]15407[/spell] altogether (though of course, if you've got no procs, use [SPELL]15407[/SPELL]). It's difficult and somewhat stressful to keep both DOTs up on every target, but it's very rewarding when your DPS skyrockets because of successful multi-dotting.

General Tips:
-Use your Shadowfiend/Mindbender as much as possible. Try to align it with trinket procs, potions, Heroism, or whatever is a nice boost.
-With Symbiosis, you'll get [spell]740[/spell]. Use it if there's some huge raid-wide damage going around!
-Do your best to cast [spell]15407[/spell] to its full extent; that is, don't clip it. This is fairly important if you're clipping your channels frequently.
-Utilizing trinket procs, Heroism, and other such buffs is key to maximizing your DPS. Pay attention to what kind of boosts you get, and refresh dots appropriately.
-[spell]108968[/spell] is a great way to save the day if your tank is about to get flattened. Keep an eye out for when you can don the mantle of Hero.
-It was probably Gren's fault.

Self-Amusement Section

[spell]43274[/spell]
[spell]32111[/spell]
[spell]5[/spell]
[spell]39172[/spell](mixed messages here)
[spell]5025[/spell]

This is pretty much all I can offer for now. Much of this is taken from the extremely useful guide at http://howtopriest.com/viewtopic.php?f=19&t=1748&start=20
I'll try to keep this updated if it's being used, and I hope it helps.

[MaddenKage]

ty for the info

[Lord Entropy]

Great guide, however, I think Shadowolf will make you pay somehow for the New Jersey comment!

Shadow is my off-spec and I'm not really great with it, so this helped quite a bit.  Refreshing dots at key moments is something I'm not doing.  Of course, my gear is almost all for healing and with my spirit I have over 20% hit I think.  Bring on the level 100 bosses!

[Vengeance]

Yeah, optimizing your stats can be the difference of a few thousand DPS. I try to get as close as possible to 15% hit, so I can maximize haste/mastery/crit etc
I live in New Jersey, so I retain the rights to insult it!

[Shadowwolf]

I hate NJ despite being born and growing up there. Tis why as a teen I would find any way and excuse to escape to Philly or NYC and as an adult why I moved out to other states still in search of my "home".

So by all means, insult away.