While size is the main focus, there are also a few other improvements being toted in the Sprint -- it sports a micro-USB port for charging (changed from mini-USB), a softer cable between the buds, an inconspicuous notification LED, enhanced call quality and the multifunction button has been repositioned to the top of one of the earpieces (the JF3's is on the outside).
Connection to your computer relies on a proprietary port -- similar but slightly larger than the iPod's -- to USBcable, which may suggest that we will see some more peripherals made for the Mini 5 and its future siblings.