Generally it needs to have (in addition to cameras, the mic, wireless networking, and the accessible bootloader):
1. easy-access expandable storage (e. g. dedicated uSD card slot)
2. 1-2 high-speed full-feature USB port(s)
3. 3.5 mm audio jack
4. an ergonomic, i. e. flicker-free display
5. a user-exchangeable battery
6. Batteryless operation mode
Additional (personal) niceties:
1. Active stylus support (EMR preferred)
2. Pogo pins for attachable keyboard, etc.
In other words: Ultra-mobile GP computing (and not telephony) is the first customer.