Imbalance is okay. It could even be stated that variation itself has in it some imbalance.
That's why I've always said WoW never truly solved any of EverQuest's issues. They just removed mechanics that caused the issues. Removal doesn't equate to a solution. If you remove social dungeons for instancing, you didn't solve contested spawns: you removed the mechanic that led to contested spawns and used a wholly different one.
WoW never solved the class issues in EverQuest. They removed mechanics and replaced them with different ones. Hollowing out the meaning of what a class is through total homogenization, making a chosen class not mean anything in the context of the world, by giving every class the ability to perform every role didn't fix the issue of not having a healer available for a group, for example. It turned all classes into fitting all roles so anyone could be the healer. That isn't a solution because you've now removed the idea and choosing of a class, what that class is. The WoW design team rode on the coattails of the technical and engineering talent all the way to the bank.
Let's be clear on what uniqueness is, let's be explicit: uniqueness isn't a different graphic or animation for the same mechanic or ability shared among classes. That's not unique, but window dressing. Uniqueness means the effect has someuniqueeffect on the world that only that class has. WoW had much, much less uniqueness in classes than EverQuest, and their "solution" to the no healer available problem led to this new problem, that of a class having no meaning.