1 - Reliability & Durability
It needs to turn on every time and be able to handle being dropped, getting cold and wet etc.
2 - Simplicity
Flashlights with too many modes, or requiring complicated manipulation of a single switch (Streamlights Ten-Tap) just get in the way of functionality. Ideal setup for me is a tail cap ON/OFF switch and side button to toggle through the modes.
3 - Commonality
I much prefer lights that use a standard sized battery rather than a proprietary one, even if the light is externally rechargeable. I already have extra batteries that can be swapped in out in the field or camping etc where the light can't be readily recharged or you need to use it immediately.