I imagine that the UI changes since Sequoia regarding the giant buttons and whitespace are because Apple plans to release a touchscreen laptop (most rumors say it will be announced tomorrow) so they've decided to make the UI touch friendly at the expense of bloating the UI for everyone who doesn't have a touchscreen. Microsoft made this same decision with the Windows releases after 7.

The rumors also said it would be announced today, so we'll just have to see. Even a broken clock is right twice a day, after all.