The Killer App for Electronic Paper: Dynamic Bumper Stickers. A single bumper sticker, neatly placed on the back of your car, hooked into a panel on the dashboard. The panel lets you download designs via, eg, USB or Bluetooth; you select one to suit your mood via the LCD panel.
This is a killer app mainly for enabling you to send brief and informative messages to other drivers as needed, such as "Turn down your brights!" or "Stop honking at me -- there's a No Turn on Red sign!". (Or something less polite.) Once every few weeks, I find myself desperately wishing I had something like this.
(Yes, I know about Road Rage Cards. But hey, the high-tech solution would be so *cool*...)