Just as a disclaimer, I am not really into PvP that much, I just had some ideas on how to balance PvP that I wanted to share with the community.
Ask yourself, what is an example of a simple game between multiple people that is perfectly balanced?
My answer to that would be rock-paper-scissors. Provided you don't have some psychological advantage or something, you have an equal chance of losing, winning or coming to a draw every time you play. So what is the problem with rock-paper-scissors? It gets boring fast.
So now lets expand the discussion to WoW, which in theory isn't as boring as rock-paper-scissors. How do you balance something that is vastly more complex and still keep it fun and interesting? That brings me to my idea, basically a "class wheel" of sorts, where you are balanced to be better than some classes, equal to others, and worse than others.
Hopefully this image will explain what I am proposing: http://i.imgur.com/iNJlC.jpg
Basically, your class is in the bottom black circle. There is 1 other class that has no real advantage over you in a 1v1, players of equal skill and equal gear will win/lose 50/50. There are 2 classes that you have a slight advantage against (win/loss is 55/45) and 2 that you have a slight disadvantage (45/55). Then there are 2 classes that you have an advantage against (60/40) and 2 that you have a disadvantage against (40/60). It is intended that there are some classes you do well against, and some that you don't. This is because if we were all 50/50 with all other classes, then it would be like rock-paper-scissors but everyone is a scissor with a different colored handle. Those differences and advantages/disadvantages are what make the game fun, and what make playing your class interesting. You wouldn't loose to your counter class every time, it would just require some more luck or skill than against the class you counter.
So let's put an example to this: http://i.imgur.com/UNH4A.jpg
Say I am a Warrior. I do well against Rogues and Druids because they just can't hang with a bloodthirsty savage with axes. However I don't do well against Mages and Warlocks, because they can keep me at bay for a long time and my armor doesn't stop magic. I do okay against Paladins and Shamans because they don't have very many tricks to keep me at bay, but they are better equipped than Druids and Rogues to go toe-to-toe with me. I don't do great vs Priests and Hunters because they can hit me from afar, but they don't have as many tools as Mages and Warlocks to completely lock me down so I can manage. Against a Death Knight, it's a coin toss, there really isn't an advantage either way.
Now the flip side, a mage: http://i.imgur.com/ZcEST.jpg
I won't break it down as detailed, but you can see how the mage fares against the other classes.
In essence, you are grouping classes into a color (black, blue, purple, gold, brown) and making that an option in the PvP "rock paper scissors" game.
It gets really interesting when you start talking about group play. On one hand, having a balanced group means you will have somewhat of an answer for every composition you face. However having an unbalanced group means you will totally wreck certain comps (because they may only have 1 guy to counter your stacked class) but get totally wrecked by others. It's nice because stacked comps will be fun, but they won't get above a 50% win ratio so they won't be near the top of competitive PvP.
Every time there is a patch/expansion, Blizzard would evaluate as much as possible beforehand, and maintain during to ensure the classes fit their intended position on the balance wheel.
Obviously where I put the classes might not be the best, but you get the idea that each class has a role, and classes it pwns and classes it fears.