Since my 4th Book of Worlds text is laying up in dry dock until its recipient gets ready for it, let me record here a few ideas for possible inclusion into it.
In Starsong Systems, my "stalled just short of the Singularity" super-tech future, the governmental systems could use a bit more explication.
1. As Dr. Rudy Rummel notes, Democracies don't make war on each other. This is as close to the Law of Gravity as it gets for Sociology. And there is also a Commercial Peace effect, in that it gets extremely unlikely for a Democracy (or indeed any gov't type) to fall if its average income its 6,900 or so per year for its citizens.
2. But I contend that this will change when we get to the levels of wealth of Starsong Systems where the rich are able to, in a cartel, stellar-form a whole solar system. One of the greatest costs of a society is to change its government. But the hypothetical Starsongians will have perhaps an average wealth of ten million dollars per person.
They can afford to experiment with their wilder fantasies.
However, the more mature and sophisticated members of this society are going to deal with this in three ways. 1) A highly decentralized governmental structure, similiar to federalism, but only more so. 2) A non-negotiable, whatever the system of gov't, and more extensive than America's, Bill of Rights. 3) Certain gov't forms will be regarded as toxic, or as strange attractors likely to provoke the general society to go down well-known dead end paths. This will provoke the uber-civilized version of "War" and "Invasion" that the Starsongers practise on each other.
One such dead-end is that it is fully in the capacity of the citizens to arm everyone with power armor, and automatic fire nuclear grenade launchers with probably superfluous targetting missile capability. One such guy could unless hit with a direct hit by a large nuclear weapon, could defeat the entirety of the American armed forces in 2005.
The likely end result of building these weapons is generalized war, and absolute destruction of everything. Since that end is unacceptable, a societal taboo (along with other persuaders) is enforced against WMD, and even ranged weapons. So, while Starsongians are still the same nasty, irritable species they always were, they now get out the hi-tech swords to do unto you before you do unto them.
Another dead end is a society in which everyone is genetically inculculated from birth with a certain viewpoint, and the ability to doubt is removed. This is considered a moral perversion of the first order.