My model is that these revolutions created a power vacuum that got filled up. Whenever a revolution creates a power vacuum, you're kinda rolling the dice on the quality of the institutions that grow up in that power vacuum. The United States had a revolution, but it got lucky in that the institutions resulting from that revolution turned out to be pretty good, good enough that they put the US on the path to being the world's dominant power a few centuries later. The US could have gotten unlucky if local military hero George Washington had declared himself king.
Insofar as leftist revolutions create worse outcomes, I think it's because since the leftist creed is so anti-power, leftists don't carefully think through the incentives for institutions to manage that power. So the stable equilibrium they tend to drift towards is a sociopathic leader who can talk the talk about egalitarianism while viciously oppressing anyone who contests their power (think Mao or Stalin). Anyone intelligent can see that the sociopathic leader is pushing cartoon egalitarianism, and that's why these leaders are so quick to go for the throats of society's intellectuals. Pervasive propaganda takes care of the rest of the population.
Leftism might work for a different species such as bonobos, but human avarice needs to be managed through carefully designed incentive structures. Sticking your head in the sand and pretending avarice doesn't exist doesn't work. Eliminating it doesn't work because avaricious humans gain control of the elimination process. (Or, to put it another way, almost everyone who likes an idea like "let's kill all the avaricious humans" is themselves avaricious at some level. And by trying to put this plan in to action, they're creating a new "defect/defect" equilibrium where people compete for power through violence, and the winners in this situation tend not to be the sort of people you want in power.)
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A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site!
Once a post gets over 500 comments, the site stops showing them all by default. If this post has 500 comments and you have 20 karma, please do start the next welcome post; a new post is a good perennial way to encourage newcomers and lurkers to introduce themselves. (Step-by-step, foolproof instructions here; takes <180seconds.)
If there's anything I should add or update on this post (especially broken links), please send me a private message—I may not notice a comment on the post.
Finally, a big thank you to everyone that helped write this post via its predecessors!