Its obvious Bankai have multiple aspects for the user, yet we never see Ichigo's real abilities. The moon could have renewal, reflection, sense the enemies thoughts, control the tides, ect. The foreshadowing: Yhwach gaining power over black, Ichigo's power 'fading to white', Kenpachi's continually released shikai (the only other continually released shikai), "the blade is me", and Ichibei's resurrection never actually materialized.
The awesome fights with the captains make the dud final battle that much more of a let down.
I figured Ichigo pulls in the surrounding reatsu when he uses bankai and uses it to enhance his abilities (which neatly combines Quincy, Shinigami, and Hallow). That would explain his rapid growth and be his trump card in the final battle. (His ability actually makes him stonger at the end of the battle than the beginning at his opponent's expense.)
P.S. It has to be a kick in the balls when they decide to wait 10 years in peace time to make you a captain after you use bankai in battle and after promoting others who didn't accomplish anything.