In many previous comments, people call on professionals to answer questions about specific fields, like physicists, neuroscientists, economists, or computer scientists. There are many people in all these professions on this site willing to respond to questions, but most of the time none of them happen to read that comment.
As a way to fix this, I propose that people well-educated in certain fields volunteer to make an "Ask an X" post where they list their credentials and specialties, and anyone can ask questions about that field. Obviously, this would also be a good place to have a discussion between professionals in that field.
Another possibility is to ask people who don't mind being asked random questions to volunteer to be part of a list that can be posted to the wiki. Then, people could just PM that person directly.
The Freenode and Efnet IRC networks have reasonably active math/physics/chemistry/neuroscience/... channels, with some of the channel regulars being experts in the area (often grad students and faculty). We've been trying to keep the Freenode ##physics channel in a decent shape for the last several years, but YMMV, depending on the area of interest, the time of day and random chance. More often than not, people just drop by for homework questions, but there are also plenty of more general discussions. The downside is that very little of it is posted online.