Just a quick post today, next time there will be more Divine Bard!
I have a theory, and to some extent it is based on a mix of 3.x and 4e D&D. Basically its that the Power Sources can be grouped into three different types: Given, Taken, and Self.
Given sources include Divine and Primal, and are granted to them by some other source such as a deity or nature.
Taken sources include Arcane, and are taken from their prospective planes, the universe itself, the mana flow, etc. without any being's consent.
Self sources include Martial and Psionic, and are from within the user, built up from training the body, mind and spirit.
I hope to add the future power sources to this list, Though I assume that Elemental will be a 'taken' and shadow may be a 'taken' or a 'self'.
Does that sound about right? Comment and put in your opinion. I dare ya!