I see the policy and promotion changes as ways of forcing the player to make a decision "now" rather than allowing him to punt till later when the "right" decision is more clear. It'll prevent policy point hording and also make decisions on early cultural buildings more nuanced as you don't want to end up with a bunch of early policies that don't help or, worse, hinder at the end.
My guess on the defensive building cost removal is that it probably effects AI decision making, thus we are likely to see stronger cities that'll be harder to take with a handfull of weakened horsemen
