Good question. Actually, I don't recall what I was thinking when I separated the two.
Probably "ideas" is a more general thing, to discuss rough concepts, which might not necessarily be feature-related.
Feature requests should ideally be specific features, and we should try to discuss the design of the features as much as possible.
You will notice we don't do much moderation. We have recently re-ordered the boards and sections. It is actually working much better now. You should have seen what we had a few months ago!