That really depends on how you mean “balanced.” Blizzard has stated several times that classes aren’t intended to be balanced in 1v1 situations, this would naturally mean that small groups won’t be perfectly balanced against every other group combination. A different ruleset means it’s a different game, besides they have several means to adjust PvP seperately from PvE (such as resilience and talents/abilities that have little use in PvE/PvP) and even do bend many rules in arenas such as long cooldowns not being usable and the new rule about switching gear. The nature of the game means that sometimes the deck will be stacked against you.
But getting back to the original subject, that’s exactly what this nerf is, a nerf that only affects pvp. From my perusal of the item database (probably not exhaustive though) you can’t even get resilience on world drops or raid/quest rewards.
I don’t know that Blizzard has ever stated that the game is intended to be balanced. I don’t believe it’s possible, even in the pve environment, and I think they know that. There’s always going to be classes/races/professions that are better/worse in a given situation. And in my opinion, there’s nothing wrong with that. Hunters are easier to level than warriors. So what and so be it.
I know, you did say “not exhaustive enough” but you can get resilience on PvE items, I picked up a ring for my paladin in Old Hillsbrad that has Resilience on it. It can be a useful stat for tanks to help become crit imune, but +defense is better.
Speaking as someone who’s been raiding as a warlock for a while, I’m convinced that a class style that favors damage over time is inherently frustrating to fight – especially when that class has a crowd control that doesn’t necessarily break on damage. And you can die from those spells even after you’ve taken out the warlock. It can be downright humiliating to watch your character run around mindlessly, then die. This is why priest mind control got the nerf hammer.
But this doesn’t make the warlock overpowered, so much as it makes it difficult to kill him without taking a lot of damage in return. That’s what people don’t like. While other classes can be shut down, a warlock is not going down without raising your blood pressure first. On account of their position as historical underdog (believe it or not), they’ve cultivated some determined scrappiness. You will be sent running, and you will probably be set on fire for good measure.