A few of the main problems with warlock DPS right now are:
At wills - Currently warlock At Wills are at least 30% behind every other class in At Will damage. All of which do around 100magnitude per second or better. Hellish rebuke for example has a cast time of 0.7seconds, deals 30 magnitude + 25% buff from Hellfire Expertise. and a further 25% damage overtime OR upfront if using NPNM. for a total of 49 magnitude per hit. or roughly 67magnitude per second and this is only with a class feature .
Lack of Burst - other than Killing Flames, every other ability is either low damage with short/medium CDs, Damage over Time, or has prerequisite conditions to work properly.
Lack of any real choice - Most of the class features are useless or extremely niche and only useful in very select situations. Same with the Feats and there really isn't all that much interaction between the feats and other aspects of the spec.
AoE - AoE is mediocre at best,
Fiery Bolt is a joke, 156magnitude single target and 78magnitude to all other targets. its pretty much useless except to apply curses for a Parting Blasphemy Build.
Hellfire Rings radius is too small and its damage over time is currently Bugged and doing 30-40% less than it should.
Infernal spheres radius is too small and regularly gets stuck when trying to trigger the explosion, requiring either all charges to be consumed by taking damage or manually slotting a new encounter power and back again.
Arms of Hadar is terrible. damage is ridiculously low and its cooldown increase per use lockout doesn't sync with 20% CD reduction from Hellfire Expertise so you have to delay using it to reset the timer, which totally defeats the point of any CD reduction.
Blades damage is OK, but it can be interrupted/removed with a stun/prone losing all damage and putting it on CD
Curse bite is still slow. regularly loses a charge even when the enemies are dead before the cast is complete. its recharge speed is not effected by cooldown reductions from Feats or Equip bonus's.
Lesser Curse - Damage is dreadful. It does 8 magnitude over 8 seconds, ( 4 ticks for 2 magnitude each) it may as well be 0 when compared to the Wizard equivalent of smoulder which is 80magnitude over 4 seconds.
Soul Investiture - Takes too long to get to 5 stacks, and its duration is too short so it regularly drops off even on boss fights that have short transition phases.
Soul Puppet - deals 45magnitude every 1.25 seconds (36magnitude/sec) cannot gain combat advantage, and Crit severity fixed at 50% ( this was recently an undocumented bug fix since it wasn't working before)
Soul Sparks - Certain bosses still do not generate Soul Sparks when attacking them. such as Valindra - the last boss in SoT. The Giant Crab in MEs and most recently the Last boss in Tales of old. there are probably others that I cant think of or haven't tried for a while.
So some possible solutions.
Increase at will damage by 30% across the board
Lesser curse - Deals 40magnitude over 8 seconds (50magnitude with Hellfire Expertise).
Soul Investiture - Reduced to 3 stacks and 33.33% increased soul puppet damage per stack. Duration is increased to 30seconds and an additional 5seconds per stack for a maximum of 45seconds.
Soul Scorch - Deals 10 magnitude per spark. Max 15 sparks consumed and 5magnitude per spark radiating damage. applies lesser curse to all targets hit.
Shadow walk - Increases movement speed by 10% and increases stamina regeneration by 10%
Dark Ones Blessing - When a target effected by lesser curse Dies you gain 1 soul spark and 10 action points
Dark Prayers - When hitpoints are below 30% you gain 5% damage reduction
All consuming Curse - At Wills apply lesser curse and deal 25 magnitude damage. Can Crit and can deal Combat Advantage Damage.
No Pity No Mercy - keep the same
Flames of Empowerment - increases damage by 2% per stack. stacks 3 times
Deadly Curse - Increases Lesser Curse damage by 100% Curse Consume no longer removes Lesser Curse but consume mechanic is unchanged
Dust to Dust - Increases AoE damage by 10%
Double scorch -Soul Scorch No longer consumes soulsparks. Deals 200 magnitude to primary target 100magnitude radiating damage but has a 15second cooldown
Power of the Nine Hells - Daily powers Grant 3 stacks of Soul investiture and refreshes duration
Parting Blasphemy - all Curse Consumes deal an additional 100 magnitude and Critical hit chance is increased to 100% for 4seconds
Warlocks Curse - stays the same
Risky Investment - Soul Investiture grants 5% to all damage per stack
Soul desecration - Soul puppet will no longer disappear, each attack generates 1 Soulspark and deals damage 50% damage to all targets within 10feat
Creeping Death - Each attack applies Creeping death Which deals 10% damage of that attack over 5 seconds. each time creeping death is reapplied to the target the previous creeping death is consumed dealing all remaining damage instantly.
Executioners Gift - Same
Soul Spark Recovery - each Soulspark Spent grants 1% recovery speed for 15 seconds and 3 action points. double scorch assume 15sparks used.
Wrathful souls - same.
Killing Flames - same
Vampiric Embrace - 240magnitute (300 with hellfire expertise)
Dreadtheft - 340 magnitude ( 425 with hellfire expertise)
Hadars Grasp - 200 magnitude 280 with curse consume (350 with hellfire expertise)
Curse Bite - 200 magnitude (250 with hellfire expertise) 12second cooldown
Fiery Bolt 160 magnitude to all targets within 15feet (200 with hellfire expertise) 12 second cooldown.
Infernal Spheres - 40magnitude per sphere (50 with hellfire expertise) 20 ft radius explosion. 12 second cooldown.
Blade of vanquished Armies - Cooldown starts immediately. cannot be removed with stuns/knockdowns. Deals 20magnitude to all enemies within 10feet every 0.5 seconds for 6 seconds(25magnitude with hellfire expertise) 18 second cooldown
Arms of Hadar - 100 magnitude (125 with hellfire expertise) 2 second knockdown. 10 second cooldown. remove cooldown increase.
Hellfire Ring - 80 magnitude initial hit (100 with hellfire expertise 200 magnitude over 8 seconds (240magnitude with hellfire expertise), increase radius by 5 feet
Accursed souls- keep the same
Brood of harder - keep the same
Tyrannical Curse - keep the same but fix the split damage so it doesn't deflect
Gates of Hell, reduce to 500 action points
Flames of Phelegethos - 1200 magnitude (1500 with hellfire expertise) radiates 20% damage to targets within 15 feet.