Balance starts with maths. Thats as far as you can reasonably take it. Imbalance is when the math is wrong, but is often mistaken with player skill differences.

SC2 is a good game to see skill imbalance taken as game imbalance. If you're familiar with blink stalker micro there are games where a protoss player can totally decimate an opponents army and not loose a single unit.

ROOTDestiny a week or so ago used a smurf account to prove that all the little things like scouting, and army composition, tactics and the like didnt mean squat till you had your base mechanics down pat. He even told his opponents that he would beat them using a specific unit and walked all over his opponents from bronze to the platinum league using zerg and a mass queen only army.

Same for WOW, look at some raid logs and see identical classes/gear have wide variances.