1 Change Pin : AREF pin plus next two I2C pins (SDA and SCL, just Analog 4 and copy 5 , not additional I2C interface ) , in addition, in the next RESET also added two pins , one is IOREF , allowing expansion board to adapt the onboard voltage , this pin expansion board just told what is currently onboard voltage ( such as UNO is 5V, can be seen as copy power pins , does not provide power to pull level on ) , and the other is the future stand placeholder pins.
2 more stable RESET circuit . RESET key position has changed, has been moved near the USB interface board corner , more convenient press.
3. ATmega16U2 instead 8U2, this does not mean there are 16K flash of R3 make your code run faster , this update is the USB interface chip services , in theory, it allows UNO can simulate USB HID, such as MIDI / Joystick / Keyboard.