"What's wrong with Apple's multi-monitor support?"
One thing immediately comes to mind: configure a multiple (3 or 4) monitor setup (again, not exactly exotic) and then full screen a video in one of the monitors ... now change focus to a different window in a different physical screen. Fun ensues. Different fun, depending on OSX release.
USB or bluetooth tethering is great - I'd prefer to use that always - but some use-cases demand a physical thing stuck into the laptop and the plain old USB A-type is flexibility I don't like to see deprecated. I'll gladly take 1mm additional laptop thickness in order to retain those and I think the 11" macbook air, with one type-A on each side, is a pretty powerful swiss army knife.