This thread is not about numbers, it's about mechanical and playstyle issues with the class that make it less enjoyable, less engaging and less desirable to have around. There are in my view 6 key points.
Specs over-specialised
Talents exacerbate this
Trap talents/glyph combinations
Bad talents and incoherent comparability along the row
Lack of utility
Lack of encounter engagement
Firstly, being overspecialised to the point of forsaking the capacity to fulfil other tasks forces a lot of spec changing that other classes need not deal with. Affliction, notably is unable to deal direct damage or AoE effectively, while Destruction is unable to function where high mobility is required.
The idea of some of the talents seems to be that they can perhaps compensate for these shortcomings - KJC for mobility, MF/Cataclysm/CR for AoE for example - but instead the obvious choice is to pick the spec that covers the shortcoming, and the talent that compliments the spec's main strength necessary for the encounter. That of course amplifies and widens the gap between 'right and wrong' talent/spec choices per encounter, and as balance occurs towards the 'right' end, and means the 'wrong' end is pushed that much further behind. In turn, that 'forces' respeccing into specialisation and talent combinations a player simply might not find enjoyable. Further to that, as new content opens, stats inflate and more gear choices become available, the need for off-spec items increases but are unlikely to be readily available when it matters during progress.
Then there are trap talent/glyph combinations - Sacrifice doesn't work with Servitude, Glyph of Demon Training works with neither.
There are bad talents. With the introduction of Cataclysm, Mannoroth's Fury is now redundant, especially after the Rain of Fire nerf. KJC lacks the flexibility of comparable talents like Ice Flows, or the multi-glyphable Spirit Walkers Grace, making it incredibly niche. That it sits alongside AoE and raw DPS increasing talents make it even less desirable. Charred Remains, as a DPS loss on going untalented against Single Target would be very difficult to gauge as persistent sustained AoE simply doesn't happen, and AoE phases get progressively less significant as gear improves. Demonbolt on the other hand is simply not in any way intuitive.
Lack of utility since Healthstones are now superceded by the more potent Health Tonics and Shieldtronic Shields, we're left with Gateway which is incredibly niched to 1, maybe 2 encounters a tier. Even there, it's unpopular with a lot of raid leaders due to its obstructive size and players 'misclicking' it.
Tying in to the lack of utility, it's too easy for Warlocks to pass on engaging with encounter mechanics with 'I'd need to respec', or 'I'd lose DPS by switching pets/talents' or simply 'I can't, I don't have the tools to'. I honestly think that not just makes the class less useful to have around, since you can't rely on them to fulfil basic obligations like interupts or purges, but also less fun to play because you're losing out or missing out if you actually want to.
Those for me are the objective issues that are hurting the class.