Actually, I've analyzed this one in school, and the conclusions the class came to were a bit differant.
Generally speaking Star Trek is more of a liberal neo-facist fantasy. The thing is that we mostly get to see the show through the eyes of "Star Fleet" who are basically the elite of The Federation. The selection process is such that only like the best person or two from a planet gets to attend the Academy each year, and out of those people graduation is not guaranteed (Wesley Crusher failed, and whatever you think of him, the character was a ridiculously smart genius... the "Davinci Of Matter and Energy" or whatever). Life in Star Trek for those who don't manage to make it into that elite is basically as a serf. Citizens of The Federation come in two major varities, one are the jumpsuited worker drones you see running around on the nicer planets like earth, living an almost 1984-utopian existance, and then people from fringe worlds like the one Tasha Yar was from (and like the planet Picard visited disguised as a mercenary) that do not play well with the rest of the federation.
Pretty much The Federation gives people a job, and sends them out as a good little drone to do it. If they don't, well typically they wind up in some dystopian hellhole, starving to death, and running from rape gangs and the like. It's very much a socialist "Human Hive" type society in the way it functions.
The whole "we are the world" vibe seen on Star Trek sort of glosses over the simple fact that there really aren't any diverse cultures. What basically happened was that there was a huge war on Earth, it was almost totally destroyed, a facist military goverment rose out of the remnants of the US, and using drug-enslaved soldiers proceeded to conquer and enslave the remnants of humanity (this is shown in "Encounter At Farpoint"). The society however softened over a period of time and turned into what we currently see as "The Federation". It's not really the descendant of any kind of peaceful merger and resolution of problems.
One fun fact is that apparently the "mirror universe" is a universe where the original facist goverment of The Federation never changed.
If you live in "Star Trek" pretty much your life is probably going to involve being a farmer (like Picard's brother), living in an infested ghetto, or acting as an administrator.
The Federation also has a bit less of a "live and let live" policy than it might at first appear. While the protaganists in Star Trek tend to view "the Prime Directive" in an ethical light, the bottom line is also that there is little to be gained by dealing with societies that are relatively primitive. The idea is to not waste resources on them, and then return when they have something to offer (so to speak). The standards for contact have been mentioned as when they develop warp technology, at which point The Federation (who observes them) sends an envoy and while perfectly diplomatic about it, usually winds up pressuring them into joining The Federation, which then pretty much takes access to whatever of worth it might develop from that point onwards, in exchange for some basic uplifting.
It's worth noting that while differant planets/species have their own goverments, you'll also notice that they get absorbed into the Federation meta-culture. Races like the Bajourn, Trill, etc... all also sport the same basic jumpsuits as other citizens (with rare exception) it's very much an anti-individualistic culture except for the elite.
Going on further would make this too long, and I'm getting a bit repetitive, but the point here is that from a lot of things I've seen, read, and discussed, there are reasons based on a lot of the details dropped, and notes left by Roddenberry, and other things why Trek tends mostly to focus on a specific type of people.
I understand how a lot of people perceive it, but I think quite a few people that have dug into it critically (like surprisingly when we did this in class) came to very similar conclusions.
I mean it can be fun to watch, but when you really think about it, it's probably one of the universes you'd least want to live in. By the numbers all the cool stuff isn't for you.
Also I believe there were some cracks in a few of the books (which are admittedly not the canon the TV series is) by third parties about how The Federation winds up assimilating people just as much as The Borg do. It's just that they do it socially.
