Here's an idea. How about Uber allowing tips on the app, but make it anonymous (driver only know who left a tip or not) and the driver only receive a lump sum tip weekly or something? This way, less tension between the drivers and the passengers, in theory.

I know it could sound good in theory but can get awry in practice (a pax can leave 1 cent tip just to improve his/her pax rating), but it is something I thought about. Who knows? Some conscientious/kind pax may leave large tips to balance things out?