Recording is what will set this device apart. I know that's not the focus for the majority of Rockbox developers but hear me out:
The iRiver H1x0 is rare in in that it has an optical input. This allows it compete with devices that cost $500. There is nothing on the market that competes with it in terms of versatility for the recording hobbyist. Why iRiver discontinued this product type is beyond me.
If you support high quality digital input and have robust recording this unit will be very popular.
It will fill a void that the H1x0 series left behind and this niche is currently not being serviced by any company's products AFAIK.
So my list goes like this:
1) Runs Rockbox
2) Small physical size - pocketability
3) Tactile buttons (I agree with pondlife, the gigabeat cross is finicky)
4) Nothing that protrudes too much from the case. (this was a flaw with the H1x0's joystick)
5) *Silent* storage (no moving parts!) CF?
6) Long runtime (always nice)
7) Integrated mic.
9) Digital-In (preferably via mini-TOSLINK)
10) RTC (for time-stamping recordings)
11) Charging from a regular mini-USB port 5V (no adapter necessary)
12) *Fast* display, good for peakmeters. (the H1x0 display is too slow)
I am indifferent to the replaceable battery idea.
I am indifferent to color display.
If a CF adapter (or similar) could be used I would be fine with HD. (I'd just mod the RockBoxPlayer for CF of SSD or something)
Just don't leave out digital input!
(24bit/192kHz would be ideal but 20bit/48kHz is fine too)
[NOTE] The name of the jack in question should be mini-TOSLINK