Generally speaking mods close questions for quality reasons or because they don't fit the format of the site. The F.A.Q. is fairly explicit about what SO is for and while I can only speak for my self, I do my best to adhere to those rules when closing questions.
As for locking questions, that is a bit more subjective but in most cases they are locked because they are well known and they have been solved. Being that there is no need for additional answers they are closed such that you don't have the spam issue of "Thanks" posts.
I doubt that there is a systemic pattern of censorship and rather what you see is the normal operations of a very large and famous Q.A. site with a very specific purpose.
I've been on SO for a while and have a good amount of karma there. I think another site couldn't replace SO for these closed questions, (there's quora, already, anyway). There is some culture around this: you can post a subjective, or even funny question, and get away with it for a bit, depending on your reputation on the site/who you are etc. The community produces these questions and answers, so merely using a different website purposed for this case wouldn't work, since it would require the SO community to generate the content, and you wouldn't have that.