For instance, that a clean source of energy that is both concentrated and virtually limitless will not have drastic secondary effects for the environment. I am trying to imagine such a fusion-fueled society, which magnifies the excesses begotten by the fossil fuel boom. The pumped-up consumerism would make today's consumption mania look quaint, and I'm guessing that few if any people in power would be willing to use the extra energy to have society clean up after itself -- Unlimited power added to today's political and economic culture would produce a system that chooses to do without nature (and why, little Johnny, would you want to hold back economic growth with any of that luddite "biosphere" baloney when we could live large as your favorite space colony heroes do... right here on earth?).
My 'realist', millenia-tested self brings me to imagine that direction for the Mr Fusion future, though the old me thinks it sounds like a slightly cheesy 1980s scifi short story. The big-budget version of the story has a believably fleshed-out society and ethical system that would reign in the excess before it got too out of hand.
I conclude that we need to internalize (not just learn) true ecological responsibility and the value of the web of life before we acquire anything like unlimited power.
Then again, maybe we'll all just float above the biosphere like they do in The Jetsons (though I never did see how they left things down there on the ground).