The signaling protocol for DisplayPort makes it much easier to support multiple monitors due to the micropacket design. VGA, DVI and HDMI require timing signals to be sent alongside the RGB data on their own pins. DisplayPort bundles that information into the packets, so it all goes over the same pins. A multi monitor setup with the 1.2 spec would have one cable going out of the computer into monitor 1. Then a second cable would exit monitor 1 and connect into monitor 2. No extra hardware would need to be in the laptop to drive two external displays, and switching the internal over would be trivial as well, since it's hooked in via DisplayPort instead of the classic LVDS most laptops used.

Clamshell mode still needs a keyboard or mouse around to wake the system, at least with my 2010 MacBook Air. It will wake with either a wired or bluetooth device.