It reduces many axes to one axis: right now some people may be comfortable with advanced discussions of maths but not psychology, or the other way around, so the sets of what counts as "advanced" will vary from person to person

That's a fair point, but I don't see many people on LW who are productive in math discussions but trollish in psychology discussions, or vice versa. Poster quality may be multidimensional, but seems to have a strong primary component.