Here's something someone said over at another forum that I frequent. He explains what the nature of the imbalance is while still remaining within the confines of what Lucas has said on the matter.

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The imbalance in the Force is Sidious, not the Jedi. A properly trained Jedi acts with the balance of the Force in mind. Ultimately selfless and the means justify the end. The Sith care nothing about the balance, using the Force to only augment themselves. Ultimately selfish and the ends justify the means.