Here are the things which both classes got in comparison to each other. As you can see, Affliction Warlocks seem to have been very buffed in contrast to Shadow Priests.
The Short:
1) Warlocks get much more versatility.
2) Warlocks get much more offense.
3) Warlocks get much more defense.
4) And more C.C + Regeneration.
5) And also more life drains.
6) And pets + guardians ...
7) And better AoE and multi-dotting.
Let us not go into the other insane buffs that Warlocks got for other specs, but for all purposes, we are only comparing Affliction Warlocks vs Shadow Priests, which are both mostly equal in Cataclysm. There is little love for us Shadow Priests in MoP ._.
Life Drains:
Shadow Priest (without popping out of Shadowform to heal, only using all of our life drain spells, since if you pop out of SF to self-heal, you have already almost lost the battle by wasting 6-7 seconds to heal and pop back in to reapply DoTs, while the Warlock grinds at us):
1) Use Desperate Prayer [2 min CD], 30% HP heal.
2) Glyphed Vampiric Embrace [3 min CD], 100% life drain for 15 secs.
3) Empowered Glyphed Devouring Plague, 33% heal for 6 secs.
Total heals = 30% HP heal + 100% of the target's HP if the target is destroyed within 15 secs = ~ 130% HP heal
Can only be done every 3 mins with VE ready.
Without VE, we only get small heals from DP every fight
However, most people will probably NOT get Desperate Prayer over the new Stealth talent, making our heals only ~100% of our HP
OR
VS
Affliction Warlock
1) Soul Burn: Drain Life [45 sec CD] at 5.5% total HP per tick for 6 ticks.
2) Normal Drain Life [spammable] at 2.2% total HP per tick. (Not Soul Harvest since Dark Regeneration is mutually exclusive of SH)
3) Glyphed Healthstone [1 min CD], 44% HP heal over 12 secs.
4) Dark Regeneration [3 min CD], 33% HP heal over 12 secs. Also increases all self-healing by 25% for 12 secs.
5) Cauterize Master (Imp spell), 13.2% HP heal over 12 secs.
6) Mortal Coil [45 sec CD], 16.5% total HP instant heal.
7) Dark Soul: Misery (1 min CD), 30% spell haste for drain life.
Total heals = ~127% HP heal + ~ 60% from 12 secs from drain life = ~187% HP heal (higher due to the 25% from dark regen)
Can be done every 3 mins with DR ready
Even without DR ready, the rest of spells have a CD that is so low that a Warlock can expect to spam them almost every fight and drain around ~107.3% of his HP.
If the Warlock takes Soul Harvest over Dark Regeneration, to be honest, over 15 seconds, he can use 1 Soul Burned SH + normal SH to get around 100% HP heal, to a total of ~160% HP heal for almost every fight while we must wait 3 mins for an inferior HP regen
Damage Mitigation, Spell Absorption, Spell Reflect and Pet Damage:
Shadow Priest:
1) Summons a Shadowfiend to attack the warlock [4 min CD or less]
2) Uses Dispersion [2 min CD]: 90% less dmg taken for 6 secs but cannot attack, 36% HP heal. We note that the reduced dmg does not reduce the 2.2% HP restoration on drain life, only mitigates the dmg component.
3) 15% dmg mitigation from SF.
4) Power Word: Shield
VS
Affliction Warlock:
1) Uses Grimoire of Supremacy to summon an additional pet to grind on the Priest (both already out-damages our shadowfiend) [1 min CD]
2) Instant casts an Infernal or Doomguard [10 min CD]
3) Demonic Resolve [3 min CD]: 50% less dmg taken for 12 secs, and CAN attack.
4) Specializes in Soul Link (20% dmg mitigation) OR Sacrificial Pact (1 min CD) for a 75000+ HP shield
5) Twilight Ward which scales 300% with spell damage
6) Archimonde's Revenge: reflects 30% of all damage back to the Priest for 10 secs [short CD, can't remember]
7) Glyph to summon 5 fiery Imps to attack 5 times each every 1 minute
8) If the Warlock takes Unbound Will over Blood Fear, he will have a free 1 min CD spell that breaks all CCs, and dispels all DoTs from him instantly, making us waste another 5 secs trying to reapply DoTs.
Even with this setup, the Warlock has too much damage mitigation and absorption versus a priest (and also spell reflect).
For (4), if the Warlock takes Demonic Embrace over the other 2 talents, it is 10 sec of absolute spell immunity, and I doubt that it is dispellable. This plus another 10 secs of 50% damage = 20 seconds of near invincibility for the Warlock.
Crowd Control and Instant Casts:
Shadow Priest:
1) Uses Silence [45 sec CD] for 5 secs silence
2) Uses Psychic Horror to horrify for 3 secs [45 sec CD]
3) Psychic Scream or Psyfiend for instant fear.
4) Fear Ward to nullify Warlock Fears (not horrors or stuns)
5) SW:P and SW spam for instant damage
VS
Affliction Warlock:
1) Uses Felhunter's Spell Lock to silence for 3 secs, or counterspell for 6 seconds. Can be cast independently [20 sec CD] and due to short CD, may be used twice in combat. Also, a good Warlock will always place his pet far away to avoid Psychic Scream, and prepare for a well-timed spell-lock for 6 secs.
OR even worse still, the Warlock pulls out a Succubus, with an instant stun/interrupt (Whiplash) every 24 secs, and has Seduction to gain a headstart of the battle.
2) As mentioned earlier, Mortal Coil's 3 sec horror [45 sec CD]
3) Blood Fear - instant horror for 8 secs [5 sec CD] at range
4) Curse of Enfeeblement: -25% caste speed on target [no CD]
5) Soul Burn: Soul Swap [45 sec CD]: Applies all 3 DoTs instantly on Priest
6) Fel Flame: Instantly refreshes all DoTs and extends duration by 50%, no CD, also does more dmg than our instant casts and is instant.
7) Demonic Circle: Teleport
8) Curse of Exhaustion: A good warlock will use this first to keep away from psychic scream to cast important spells like Haunt.
This one is pretty even in terms of CC, but the Warlock will always get off his horror effect first because Blood Fear has a further range, and lasts much longer than our horrors. However, he will also always get to apply his DoTs first and get the initial advantage, or blink away with teleport
Dispels:
No doubt, this will make a very huge difference. However, from playtesting, it seems like many of the new Warlock buffs cannot be dispelled. It is not yet known. However, a good warlock will always keep lesser buffs such as Unending breath and Detect Invisibility, on top of Twilight Ward and Dark Intent up. Since dispel only randomly dispels 1 beneficial effect, to dispel any of these 4 lesser abilities would waste around 6 seconds of GCD time. Furthermore, it may end up in vain because what was supposed to be dispelled might not even be dispellable.
For the Warlock's felhunter, even though it has a 8 sec CD dispel, we priests do not have 'lesser' buffs such as Warlocks, and without a doubt, the Warlock will dispel one out of 4 of a Priest's important buffs: PW:S, Fortitude, Innerfire and Fear Ward from the start, and this is off the Warlock's GCD.
Single Target Damage Debuffers and Enhancers:
Shadow Priest:
1) +20% Shadow Damage from Shadowform
2) Crappy Shadow Orb system that does not buff damage directly
3) 15% proc chance for Mind Flay to give an instant cast Mind Blast
4) Power Infusion for a 20% haste and -20% mana cast.
VS
Affliction Warlock:
1) Potent Afflictions Mastery allows an additional 300% of Spell damage to scale with all 3 DoTs
2) +40% Shadow Damage for 8 seconds from Haunt
3) +8% Spell damage from Curse of Elements
4) 10% proc chance for Nightfall to give 1 soul shard for Soul Burn (and as I type this, I just realized that Soul Burn has no CD except for instant summons)
5) Malefic Grasp (the Mind Flay equivalent for Affliction Warlocks) that causes all DoTs to tick 100% faster (which in essence, causes all DoTs to do 200% damage)
6) Dark Soul: Misery for a 30% haste on a shorter CD.
There is little contest between Affliction Warlocks and Shadow Priests. The only key difference lies in our ability to heal.
Long Term Durability:
Sure, you can say a Shadow Priest can drop out of SF to heal and outlast the Warlock, and using Dispersion and Shadowfiend to regenerate around 60% of our total mana (maybe 80% with the empowered Shadowfiend talent), but so can the Warlock. Glyph of Dark Regeneration replaces the Warlock's Life Tap and grants him a +625% mana regen that scales with more haste, with no cooldown, while our shadowfiend has a 4 min CD.