I think the development team has done a good job of choosing sub-forums but I still don't like to have them; or to clarify I think there should be fewer of them. I don't know if software exists for this, but I support the idea of moderator-controlled tagging as an alternative to sub-forums.
with this type of design, the OP would choose from a list of predefined TAGS all that apply to the topic. Searches would be filtered using the same tags. One benefit is that this would make it less likely for people to cross-post.