Here's a rundown of the main problems Warlocks have in arena right now.
1. Damage is too based around Dark Soul. With the addition of secondary resources (Burning Embers, Soul Shards, or Demonic Fury) in MoP, and the huge power of Dark Soul combined with the on-use-spellpower PvP trinket, Warlocks in PvP must save their secondary resource to use, for the most part, only in Dark Soul. The steady-state damage that a Warlock does for 90% of a game feels like a meaningless chore to get to the 20 seconds where you'll actually be able to kill something. This means that, at low levels of play, where players may not have the awareness or capabilities to interrupt the Warlock during Dark Soul, Warlocks seem very overpowered. But at the highest levels of play, where players are all over my Dark Soul, I feel ineffective and frustrated as they spam CC on me during my cooldowns.
1a. Damage is too low outside of Dark Soul. Here are some screenshots of my damage breakdown on recount after a good amount of arena games. Mostly as Destro, some with Demonology, and a few Affliction games.
Chaos Bolt is a little bit higher than it should be, as I recorded some data from hitting dummies. Most of these abilities are instant cast. Of the abilities with cast times, Chaos Bolt is mostly cast in Dark Soul. Incinerate hits for an average of 21,480, the Immolate DoT ticks for 4.3k average, and UA ticks for 3990 average. Soul Fire hits for an average of 16,786, and Haunt hits for an average of 27,233. Look at my gear. I am in nearly full Malevolent Gladiator's with the T2 weapon upgraded twice. I have 481 equipped ilvl. My gear is very good, and yet the damage numbers on all of my casted abilities are very low, with the notable exception of Chaos Bolt (which still only hit for an average of 86k). As Affliction or Demonology,opponents can simply ignore me, because the spells I cast are not threatening enough (outside of Dark Soul+on-use trinket). Or they can train me down... which leads me to point #2.
2. Lack of passive damage mitigation. Ever since the first season of arena, Warlocks have had Soul Link, which has provided 20% damage mitigation. With MoP, that's changed. We've lost the 20% damage reduction, and the new Soul Link is not a good ability. It transfers damage two ways, so that people can simply train your pet and kill you. Yes, you can take it off, but that costs a global, which is significant, and it has a 10 second cooldown, which makes the ability extremely clunky to use. I would understand removing Soul Link passive damage reduction if damage reduction was being removed across the board, but it has not been. Shadow Priests and Boomkins have kept their passive damage reduction (nearly as much reduction as old Soul Link, and no penalty of damage going to the pet). Warriors have gained 25% damage reduction for much of the game with the new Defensive Stance. I don't believe that other classes having these abilities means Warlocks need it, but 20% damage reduction was a pillar of Warlocks in arena, which has been taken away in MoP.
3. Too much instant damage, casting not rewarded. Destruction is really the only spec that casts a significant amount in PvP anymore. With Blood Fear and Soul Burn Soul Swap, Affliction doesn't cast much beyond the occasional UA. With Demonology, you can opt to cast a few Soul Fires when you have procs, but the damage (see above) is not significant at all. Blood Fear is one of the most situationally overpowered abilities in WoW's history. If you're a healer without Tremor Totem, you're gonna have a bad time. If you have Tremor Totem, Blood Fear's power is much diminished. Blood Fear artificially props up both Warlock and Shaman power in PvP, in a very un-fun way, for both sides.
4. CC options gone. With MoP, Shadowfury, Death Coil, and Howl of Terror were all put on one tier of the talent tree. In the past, Affliction had both Death Coil and Howl of Terror, and Destruction had all three (albeit with a cast time on Howl). These abilities combined to give the Warlock a bit of time to cast, but with Warlocks being forced to pick only one, we lose 1-2 of these crucial (but limited) instant CC abilities.
Here's what I'd propose to solve the issues. Ordered from least affecting PvE to most affecting PvE:
1. Remove on-use-spellpower/agi/str PvP trinkets from the game. This is more for the health of the game, rather than a Warlock-specific problem. Replace them with Battlemaster, or simply refund the conquest/honor points. These trinkets do not provide interesting gameplay--nearly every player simply macros them in with their other offensive cooldowns (Dark Soul, Avatar, Icy Veins, etc). The on-use on these trinkets might as well say: "While doing 50% more damage, you do an additional 20% more damage." Offensive cooldowns are out of control right now in PvP, and this is one change that you could easily make which would remove one stacking cooldown from every class, without affecting PvE at all.
1a. I don't expect this to happen, but I'd love to see the proc trinket gone as well. Let people pick between Battlemaster, CC-breaker trinket, and a third, spec specific trinket, which provides a passive or active ability, and is used as a PvP balance dial, much like PvP power. For example, Affliction's trinket could be a passive which increases DoT damage by 70%, (or whatever number is balanced) but removes the Soul Burn: Soul Swap ability. This would let the devs tone up or down specs in a PvP-specific way, and could let them do things with abilities which would otherwise break PvE.
2. Reorganize our level 30 talents. Mages were given Ring of Frost on a talent tier with two new CC abilities, and Deep Freeze was made baseline for all specs. I would like to see a similar thing happen with Warlocks. Give us 1.5-2 minute cooldown Mortal Coil baseline, Howl of Terror baseline (possibly with a 1.5 second cast time for Demonology and Destruction), and make us choose between Shadowfury and two new abilities for our level 30 talent. There's tons of potential here for cool new CC curses.
3. Nerf the duration of AE Fears to 6 seconds. Psychic Scream, Howl of Terror, and Intimidating Shout are very powerful with dispel on an 8 second cooldown. If Howl of Terror is made baseline for all Warlocks, I think this needs to happen.
4. Remove Blood Fear. Replace it with another ability. If removing it is not an option, make Blood Fear only have a 15 yard range.
5. Increase UA dispel damage. Right now, from memory, it crits for about 60-80k, depending on cooldowns. This is simply not enough, and with dispel instantly removing all of our DoTs and Haunt, Affliction needs more protection. Dispelling a Haunted target is currently a no-brainer. Maybe change the Glyph of UA to not remove the Silence portion of the dispel punishment.
6. Add some passive damage reduction to Fel Armor. Warlocks have, for years, been based around being a bit tankier than your average caster. Suddenly this is not at all the case. Mages with their armors, Shadow Priests, and Boomkins all have significantly more passive damage reduction than Warlocks. I know we have 10% extra healing received, but it's not enough. Give Warlocks 10-20% passive damage reduction in Fel Armor. 20% is almost certainly too much, and 10% is probably a little on the low side. Maybe 15%? If needed, reduce Unending Resolve to only 20-30% damage reduction.
6a. Some choice of armors would be great. We used to have Fel and Demon Armor, and could pick which one to use depending on the situation. Mages still have their three armors, and we lost our choice. It'd just be cool to have a more offensive based and more defensive based armor. But this is just something that would be nice to have, not a gamebreaker.
7. Add some passive self-healing back to Warlocks. Warlocks have always been able to regenerate some health simply by doing damage, yet in MoP that is gone in PvP. Soul Leech is a good ability which should simply be baseline for all Warlock specs, as nobody will ever take it over Dark Regeneration in PvP, and I believe most people take Soul Leech in PvE (correct me if I'm wrong). In the past, when I got into a 2dps vs 2dps situation in 3s, (after trading healer kills) I would be able to sustain myself for a short amount of time with some passive self healing. Now, if I don't have Dark Regeneration + Healthstone up, I have essentially no way to heal (unless I'm Destro, with Ember Tap, which is great, BTW). I'd like to see a Warlock who was playing defensively be able to gain a bit of life back, like the old Siphon Life. Yes, I know Drain Life still exists.
8. Reduce Dark Soul numbers by 33-50%. Make Destruction's Dark Soul give 15-20% crit, instead of 30%, and so on for the other specs. Nobody likes Chaos Waving for 200k crit, and nobody likes 200k Chaos Bolts. Tone down the burst a bit and increase our steady-state damage.
8a. Increase our steady-state damage. Demonology's Corruption, Shadow Bolt and Soul Fire; Affliction's plain non-CD DoTs; Destruction's Conflagrate and Incinerates. These all need to hit harder in PvP. I know this will mess with PvE balance, so I know it's unlikely. Still, I feel that something in this area needs to happen for Warlocks to be a fun class this expansion. It's not fun to simply be waiting around for Dark Soul to come up. I think that the spec-specific PvP trinkets I suggested earlier would be a good way to do this. Or have these abilities gain extra benefit from PvP power. Perhaps a simple PvP power coefficient on abilities?
8b. Tone down our hardest hitting abilities a bit. The two main offenders are Chaos Bolt and Chaos Wave. I know that Chaos Bolt was already toned down a bit with the extra Sacrifice damage being added as a DoT, and it's totally possible that the new Chaos Bolt will be fine with Dark Soul at 15% crit. I don't know. I do know that Chaos Wave currently hits too hard, and there's a ~10% chance that you crit and hit for twice that. Chaos Wave damage needs to be toned down a bit. It currently hits for 70-100k in PvP with Dark Soul + on-use trinket, depending if you have Grimoire of Sacrifice or not. If the ability crits, we're talking 140-200k AE damage on an instant cast ability with another one coming in the next global. If two crit in a row, you've practically won the game right there. It's incredibly powerful and makes Demonology a very one dimensional spec, based around gibbing someone with your Chaos Waves once every 2 minutes.