Vaderisnothayden said:

As for the Sith, logically if there's only 2 Sith and many Jedi and the Jedi are sitting pretty as the guardians of the republic and the Sith have to work in secret, then the imbalance is that the Jedi are too powerful. But I don't think that's what George meant by imbalance.

The Dark Side itself is the imbalance. Jedi and Sith aren't two equally valid viewpoints. The Dark Side is intrinsically bad. The imbalance was that the Sith existed and had been growing in power and influence for 1000 years.