The best discussion platform I know of currently is http://www.discourse.org by some of the people behind stackoverflow (though obviously it does not do many things we'd want Arbital to handle), worth at least mining their feature list (http://www.discourse.org/about/) for good ideas and examining their design choices if you're not already aware of it.
Comments
Alexei Andreev
Yup! Eliezer Yudkowsky