The functionality exists in the site... but we don't use it enough.
When you create a new thread, at the top add the tag "progression" (you can see it on this thread). NOTE - you can tag SOMEONE ELSE'S THREADS, so for example if you find a good progression thread from ten years ago, you can still tag the thread "progression" after the fact.
Then, when you do a search, click advanced search and the last tab will be "search tags". Simply start typing "progression" and the tag will pop up. Search by the tag "progression" and every thread that is tagged "progression" will come up. As people use tags and they become more popular, they will show up with greater significance in the "tag cloud" and you can click on them directly.
Tagging a thread allows the thread to show up in more than one "list". In other words, say you posted a thread about a Japanese Maple Book, you could post in the Books forum but also add a tag for "Japanese Maple" so anyone searching that tag will also see your thread.
@TreeJunkie @Brian Van Fleet @MHBonsai ... what do you think?