Actually I'd say USB experimentation is easier with the Uno board. Changing the USB code of a 32U4-based board involves sacrificing the Arduino bootloader, in which case it's no longer an Arduino and cannot run Arduino sketches. It also means you have to merge your program
It has an ATmega16U2 instead of an 8U2, and breaks out an extra four pins.
If you really want to play with USB, then get a board that allows you to play with USB in the sketchvs having to mess with the USB to serial interface chip.
Yes, true for the current version of the Eleven. However, the ElevenR3 is on its way: the first thousand have been assembled already, and they'll replace the Eleven as stock runs down. The R3 version has the R3 headers (of course!), microUSB instead of miniUSB, and a 16u2.