You can’t both be a good meeting-place for experts and a good place for novices to get expert advice and an advertising venue.

I don’t agree. There is no clear cutout between what means to be an expert and a novice. What content you’re exposed to is the output of the service’s support for user profiling and search. It is simply not possible to get rid of an important subset of your customer base without causing false positives and generate ill-will. Finally, we should keep in mind that yesterday’s novice is today’s expert.