CJ at 2012-03-29 12:32:17:
The big one, which was repeated again and again during the live tweet, was the theme of Michael's identity. Is he an opportunistic amoral "fixer" or can he be more? Can he elevate himself from the "guy you buy," to the higher evolved, "guy you kill" (because he won't be stopped any other way)? Characters repeatedly ask him, or each other, "Who are you?" or "Who is this guy?"
The other theme is that nature can renew and even save us--if we allow it to and don't smother it to death with greed and our own shortsighted plans. Arthur, bearer of bread, one of the symbol's of nature that appear in the film, became the champion of this truth but was killed for it. The image of horses—represented by images in books in the film, spoken dialogue and then by actual horses themselves--represent an avatar of nature that will ultimately save Michael's life (noted by Scott Frazier in the tweet-fest), when he pulls to the side of the road to get close to them.
Parallel with the nature theme is the thematic question, "Can we regain the good that we had in childhood, but has been worn away by time?" Henry Clayton is a figure of wisdom cloaked in childhood innocence, a figure of goodness. Michael recognizes this, even exhorts his son to hold onto it, but for most of the movie Michael can't quite regain it himself, and constantly ignores or sets aside Henry's Realm & Conquest book, symbolically ignoring Henry's message. But the elemental truths of the book touch Arthur, and seem to crystallize the predicament he's in, opening his eyes. He even binds his Secret Memo, the one that can destroy uNorth, in a red cover with a R&C chapter title. Michael wields it like a holy sword at the end, using it to finally win the war that took Arthur's life.