In no particular order of priority...
1) SPDIF Input
2) XLR Inputs with a 3 position switch; 48V Phantom, No Phantom, and Line In. Allows using powered and unpowered mics, while also allows an external preamp to use the XLR inputs as line-ins while bypassing the recorder preamp, but not the ADC.
3) Capability to be powered from variable input voltage source from a cable with a locking, readily available, NON-PROPRIETARY input jack. Unit should be polarity protected and/or capable of operating with reversed polarity.
4) Very low current draw allowing maximum field run-times.
5) Flash-memory based system without limitation to memory card size. Functional with maximum variety of card manufacturers, speeds, sizes, etc.
6) High gain headphone amp with user selectable monitoring options, including input selection switching a/b, a/a, or b/b monitoring, source delay (time offsetting monitored sound from real time sound to enable the user to better hear the monitored sound).
7) Menu selectable, rapidly changeable menu presets.
8. User selectable gain range (low range and high range, at min) on the level meter...so that when levels are lower the user can switch the scale and still have good resolution on the level meters.
9) Optional capability to power from higher capacity RC battery's via Tamiya connector.