As I get more into playing jazz guitar, I found that I am missing a lot of foundational music theory. Apparently, I never carried over any notation/sightreading that I may have had in school band, or any intellectually-aware-of-how-the-piano-keys-relate-to-major-scale, etc., and I mostly played guitar by "feel"/ear and some learned scale shapes and relative positions for chords.. but I never really put it all together or learned the ideas behind it.
Learning some music theory (like what the circle of fifths diagram on the cover of the Guitar Grimoire shows) has been like getting into a secret society. It all has a nasty learning curve (thanks western history for being confusing), but is paying off so far.
So, discussion time: What's been the most interesting thing you've learned? The most useful? What do you use all the time? What is useless?