1. DoT damage is too low. DoTs by themselves need to be a threat for affliction to be viable. When a single rejuv/riptide can completely or almost negate a full row of warlock dots, the system isn't working properly.
2. The shard system cripples Affliction in arenas. Simply put, there's just not a good way to get shards back. Nightfall is too RNG and there isn't a good passive way to get shards back. I feel like shard return should work similarly to ember generation as destro. Soulburn/Haunt can cost a shard, but the rest of the damage you do needs to be constantly regenerating shards instead of RNG granting them every so often.
3. Affliction is too reliant on casts. This is a hard topic to really discuss, because we all know that everyone wants classes to have to cast in arena. However, the current environment kind of doesn't allow that. Every melee class has multiple ways to stop casts, and has multiple gap closers. If you're not constantly casting, you're doing nothing. There needs to be a way to guarantee casts, a better/more effective gap opening mechanic, or more instant stuff (i know..).
4. Passive defense is still poor. Historically, warlocks have been able to soak lots of melee damage. In return, they had some of the best (probably the best) passive mitigation of any caster class. Passive 20% soul link, siphon life, instant death coil + howl + shadowflame on top of fel/demon armor passive DR. Now, soul link is a garbage talent that actually makes you more vulnerable to damage because of cleave effects, siphon life is not good since it depends on corruption (20% of a corruption tick that averages about 2200 damage = 440 heal every 2 seconds), shadowflame gone, you have to choose between howl and coil, etc.
*Edited out the dumb comment about demo/destro