Interesting blog, Tom -- thanks.

Looking closer at some of the multi-world systems mentioned, such as Star Trek's Federation -- and the Alliance of Firefly, which wasn't mentioned -- might reveal plenty of centralized government at the planetary level, sometimes to the point of oppression. Of course, that was a result of the autonomy granted by the overall scheme... and it allowed the writers of those shows to make further statements about the blessings of decentralization, democracy, and economic fairness.

Another thought: In the world(s) of Star Trek, especially in the post-Roddenberry era, even the empires were given cultural traits that could be admired (prime example: the samurai honor code of the Klingons). The non-imperial cultures weren't all perfectly enlightened, either: the political backstabbing behind the theocracy of Bajor comes to mind.