I have a weird answer to this, because I think the development that impressed me the most is the development allowed for by the epilogue.
When I finished the game as Gale with Astarion at my side, I thought Gale would probably love to go home to Waterdeep but that Astarion might not enjoy the settled life. So I offered to go looking for a cure with him. When opening my letter from Elminster at the party, the old wizard recognised that leaving his library to go adventuring was an unusual move for Gale to make. In short, the game had recognised character development of my Avatar, which I think is pretty cool. The epilogue /morning-after-conversation has a few of those thoughtful little moments for the Origins which I do extremely appreciate.
Oh yes and Ketheric's fall from grace is wonderfully tragic development.
