In the etcetera forum it looks like you should be able to sort on the number of replies, however if you do, it does not appear to change the order of the threads.
Susan B Just a WebGUI user, not a PlainBlack employee
This is a real bug.
At some point, sorting by number of replies was taken out, but the template variables (and likely templates) were never updated.
See Asset::Wobject::Collaboration.pm, getThreadsPaginator and getViewTemplateVars.