I've had many ideas for possible utopias, and read about many more, but I seem to always stumble on the same problems (or if I don't, someone else usually points out (correctly) that my solution for one of these is flawed):
Expertise verification. How do I know that you know what the hell you're talking about? (having members of the society all trained in hardcore bayesian rationality would help, but obtaining evidence that another rationalist has the evidence that they seem to have or claim to have is still costly, and arguing to an aumann agreement can waste tons of time depending on situation)
Scarcity of resources needs to be solved somehow, i.e. Sci-fi technology usually necessary.
Advanced cross-domain logistics involving math beyond the ken of most mortals. No, really. Counter-weighting evaluations of city design, efficient transport, short transit times, aesthetics, tribal / social proximity, local diversification, interpersonal relationships, all thrown into a big mess of predictive algorithms that are somehow supposed to take into account possible future desires of unknown people and unknown events.
Interpersonal relationships. Someone is going to seriously want to kill someone else eventually, period. None of my ideas nor the ideas I've seen so far even hint at a realistic solution. A good utopia should also encourage and help forging good social groups and meeting awesome people and making really great friends and so on... nope, still no solution there either. All my attempts at a solution for that last run into the "Wow, too hard maths" problems of logistics in the above point.
Memetic preference effects. When a certain project is really cool to work on, everyone wants to be working on that project. Just basic math and social science is more than enough to understand that making sure that enough people are working hard enough on the really important problems that need solving is hard, especially if you can't just throw money at a few of them and tell them to shut up and work. To a lesser extent, people also simply usually just want the easier or more impressive tasks and jobs, so the important but non-mentally-available or "icky" stuff (like, say, sanitation technology AKA toilets and sewers) gets left very far behind.
Assume for the time being that it will forever remain beyond the scope of science to change Human Nature. AGI is also impossible, as is Nanotech, BioImmortality, and those things.
Douglas Adams mice finished their human experiment, giving to you, personally, the job of redesigning earth, and specially human society, according to your wildest utopian dreams, but you can't change the unchangeables above.
You can play with architecture, engineering, gender ratio, clothing, money, science grants, governments, feeding rituals, family constitution, the constitution itself, education, etc... Just don't forget if you slide something too far away from what our evolved brains were designed to accept, things may slide back, or instability and catastrophe may ensue.
Finally, if you are not the kind of utilitarian that assigns exactly the same amount of importance to your desires, and to that of others, I want you to create this Utopia for yourself, and your values, not everyone.
The point of this exercise is: The vast majority of folk not related to this community that I know, when asked about an ideal world, will not change human nature, or animal suffering, or things like that, they'll think about changing whatever the newspaper editors have been writing about last few weeks. I am wondering if there is symmetry here, and folks from this community here do not spend that much time thinking about those kinds of change which don't rely on transformative technologies. It is just an intuition pump, a gedankenexperiment if you will. Force your brain to face this counterfactual reality, and make the best world you can given those constraints. Maybe, if sufficiently many post here, the results might clarify something about CEV, or the sociology of LessWrongers...