People can never speak freely unless they know that they're free to let things get a little out of hand on a regular basis. Over-moderation merely gives the illusion of a stable and friendly community by only providing room for the expression of a small subset of ideals and opinions.
With peer-moderated sites, this leads to horrible, majority-rules group-think (i.e. reddit and digg), and with sites administrated by kids, it usually leads to a strict social hierarchy with an untouchable elite and hoards of brown-nosing followers, with everyone else piled down on the bottom and treated as trash.
However, even with a professionally moderated site, as that one appears to be, the only conversation you'll find is laid-back chit chat about daily life, slightly centrist political discussion, maybe some howto discussion, and the like. In other words, you'll only find luke-warm discussion that won't rock the boat. The end result is that members learn to be careful about what they post based solely on community standards, with moderators' opinions forming the basis of those standards, instead of learning to develop and express their own individual standards built on their personal beliefs and ideas.
Just ask yourself this, "Can I disagree with a moderator and not get banned?" Or better yet, "Can I disagree with them and win?"
If the answer is pretty much "no", then the community there is probably over-moderated in some way.
