I've still yet to figure out why an ultra-thin, 15"-17" laptop style display you can toss in a backpack with a laptop that has a normal DVI/HDMI connector isn't available somewhere. I've seen some on the market, but they're way too thick and heavy for that kind of use-case. I'm talking rMBP display quality and thickness heaviness.
DVI/HDMI can't deliver power. DisplayPort and USB3 can. I think that'd be a big factor for people. I'm almost ready to buy this thing: http://www.amazon.com/ASUS-Portable-USB-Powered-Monitor-MB16... because I don't want to haul around another power brick for a second monitor