Anything that doesn't break our http://yoomoot.com/terms-and-conditions-of-use/ (basically anything that could get us into legal trouble) and http://yoomoot.com/articles/please-no-more-youtube-comments-or-how-yoomoot-reconciles-free-speech-with-politeness/ .
yoomoot's strength is in handling large and complex discussions with many tangents, so we particularly encourage questions that lead to those kind of discussions. For example, http://yoomoot.com/acm has asked some great questions.
But any kind of question is very welcome. I just can't claim that Yoomoot is any better than standard Q&A sites for them :)