> So I think it would be wrong simply to say "the UI has gone a cliff, they've just thrown away their own HMI guidelines."

My own view is that Mac OS X 10.5 Leopard was the beginning of the end, and it's gone downhill ever since.

> they at least have a somewhat logical goal in mind -- in this case, making the corners neatly fit various different kinds of window content.

I would emphasize "somewhat", because as noted, the corners do not neatly fit the window content at the bottom, which is ironic, because Apple claims that their intention is to emphasize the content, but their implementation actually clips the content!