Note: if you think the game can just be unbalanced and have one class or one spec much stronger than the others, go away. You're stupid and will have nothing meaningful to add. I don't care what I'm playing, if there are choices then those choices need to be relatively balanced. Perfect balance is not possible of course.
1. It will have unintended consequences.
Let's say we buff Big Bad Voodoo so that Witch Doctors can keep it up all the time to compensate. Oh, wait, now we've just made Witch Doctors mandatory for any Uber runs. Ok then, let's add some reduce cooldown on crits...well now we have a Witch Doctor build that can stay permanently in Spirit Walk.
Skills do not exist in a vacuum, every skill affects everything else. Even WOTB itself is only too powerful because of several skills that make it 100% uptime, remove any one of those and it's broken.
2. It will start an "arms race"
So we just buffed everyone and everyone is happy. Then suddenly all the Barbs are trading in their gear for Demon Hunter gear because the buff to Demon Hunters made them ridiculously OP (see #1). So we'll just buff everyone again...and now everyone's a Wizard.
If you just buff everyone, something is going to be overbuffed and ridiculous. Remember that WW Barbs never existed before a lot of buffs and nerfs, since they could not survive in original Inferno.
3. It will make the game too easy again
If it's well-established that for the current game, one class is too powerful and as a result that class just tears through the highest difficulty, buffing will just result in everyone being too powerful. This doesn't conflict with the "you can never have too much power" design philosophy because you still need to have some sense of challenge. Go watch well-geared Barb videos, they just kind of mow down MP10.
Games have to be at least a little bit challenging in order to be fun, and if we just keep circle buffing every time something is OP, the game will just be stupidly easy (it already is but even more stupidly easy).
4. It's just plain harder on the development side
The forums seem to think that code comes out of a magic box. In reality, it takes time and effort to code things and as anyone who has written more than a "hello world" program will tell you, any time you change your code you will have unintended consequences.
Buffing a whole lot of skills in a way that is both interesting and viable is a LOT harder and will take a lot more time than just nerfing the overpowered skill in question.
So please, stop being dumb and saying "just buff everyone else!" If a single skill is causing something to not work as intended, it needs to be changed to work as intended. You don't change an entire game because one skill is broken, you change the obviously broken skill.