Given Less Wrong's size and demographics, it doesn't seem unreasonable to expect at least one flaming wreck like the Roko situation (especially given his prior behavior patterns); can you think of an online community centered on ideas that doesn't occasionally have someone over-commit to an extreme version of one of their ideas?

I take offence on Roko's behalf (and as someone who wishes to influence norm such that the above would not be accepted if directed at me at some time in the future). Over commiting to an extreme version of one of their ideas is an absurd thing to imply. Roko was not committed to an idea - it was casual speculation. The problem was entirely social. In this instance - and in the general case it is personal behaviours, particularly of leaders, that make all the difference.