It's even true to some extent for druids. Even though one of our DPS specs is caster ranged and the other is physical melee, you still get people saying "lol feral, go boomkin rtard" when boomkin is deemed superior or vice versa. Same with shammies and ele/enh.

Yep, but in those cases it's perhaps more about ranged and melee not being really balanced than about the specs not being close enough.

Blizzard has often a quite hard time trying to get melee and ranged to be balanced. Druids and Shamans just see it closer due to having melee and ranged specs.

For hunters, it's Beast Mastery. For mages, it's Frost. For rogues, it's Subtlety. These three specs are similar in that they all belong to pure DPS classes and they all suffer from low representation. Subtlety is unlike the other two in that it's known for it's high survivability and capability of high DPS (often besting Combat) if one can master their demanding rotation. Alas, Frost and Beast Mastery isn't as lucky as Subtlety. For warriors, Arms is head and shoulders above Fury but Fury doesn't suffer like the aforementioned specs in that it has a perfectly reasonable sample size to draw from.

For death knights, Unholy and Frost are pretty much neck and neck in T13. For druids, Balance and Feral are about the same as well but you can't really compare them as one is ranged and one is melee. The same goes for shamans, where Enhancement and Elemental are both competitive but they're different types of DPS. Warlocks are unique in that all three of their specs are competitive.

As for Priests and Paladins, they're lucky and unlucky at the same time. They don't have two DPS specs for players to switch between so players can familiarize themselves with a spec without needing to worry about all that work going to waste next patch. Unfortunately, they have much less choice in their DPS.