The way Lucas has spoken about it, "bringing balance" meant getting rid of the Sith.
Yeah, the Sith abuse the dark side, creating an imbalance.
Now the Jedi just sitting around in their temple constantly indulging in the light, acting as though the could do no wrong, that did nothing to help the balance either. They allowed the dark side to thrive by assuming the Sith had been wiped out, even with evidence presented to them that suggested otherwise. Not to mention what the Plagueis book suggests, that an over indulgence in the light will cause the dark side to grow stronger in compensation.
Yoda's arc in the Clone Wars also touches on this; denying the dark side, denying your weaknesses and flaws, is arrogant, and only causes it to become stronger. Balance comes from accepting your flaws, accepting your dark side, and mastering it.