One idea I have to try to organise my blog better is to create a directory with consistent themed topics that point to relevant pages on my blog, as my current blog structure, even though I have tags and categories, is a bit haphazard. Is this a good idea? Will I see improvements in my main site SE rankings by having the structure described in a directory site?
You mean as in a Google Site Map or as in a human-readable Site Map? With over 700 pages to index, it's going to be close to a directory! Perhaps that's a good idea for a WordPress plugin - cross-reference my tags and create an automatic site map, and use PageRank/backlink count to sort within each category? Does such a thing already exist?
I meant a sitemap for humans. Wordpress has lots of plugins available for sitemaps and general navigation tools, a seperate directory is not required IMO. But you could start one and in the future it might grow to be a full niche directory. You might even make a buck or two!