Hi Jeremy,
Thanks for your response.
I don't know whether you've seen the other thread about 'contexts' and 'topics', but I think that an overuse of 'topics' as a convenient label to classify almost everything that one cannot classify in any other way (as, for instance, 'location' ) will result in fact in rendering the category 'topic' almost useless.
At least the way I understand the use of 'topic' in a GTD system is as a category that one uses to classify the major types of projects one is involved in on a day to day basis.
I have categories such as Home (for chores, home improvement tasks, etc), Research, Teaching, Shopping, etc. This allows me to review all projects related to this particular area as part of my daily or weekly review.
You can see how the addition of a lot of new subcategories that could be useful for reference will make 'topics' not very useful in the review process.
Furthermore, we don't even need 'topics' to make it easy to find items stored in reference. The work around I use for that is to include 'keywords' in the Notes field. So a particular item in 'reference' might have multiple keywords:
Keywords: research, linguistics, semantics, change, article
Using the search feature allows me to view only those items that have the particular tag the keyword "field" I inserted in the note.
Having folders is needed for a different reason. For similar reasons you group tasks into projects. You could also simply create as many topics as projects you have and that would allow you to find all the actions related to a project. Yet, it is easy to see what a great thing it is to be able to group actions by project.
I think that the 'reference' section would also benefit greatly by adding the possibility of grouping items under different folders. You just extend the system you have for projects but change the name project for a different name.
The idea of connecting projects to reference items seems indeed very interesting but it could be complementary to what I suggest. Can you tell us a bit more how you plan to implement this "connection"? I'm really interested in finding out more about that.