Similar to what everyone else has said:
Small (like a ARC AAA, or even smaller!)
Durable (ditto, internal aluminium tube with hard rubber protector outside)
Multi-lighting modes (low-med-high)
Rechargeable Li-ION battery
Runtime indicator (small multi-colour LED, Green,yellow,red, and flashing red for imminent power loss)
Clicky tailcap switch (with twist for disabling operation)
Charger base station (also small enough to fit in a pocket)
Sealed operation (dust, dirt, waterproof)
(Except when charging, the tailcap can be rotated off to expose a charging plug)
Indestructible lens (synthetic diamond?)
~1W High efficiency LED (quantum-dot dye, or super efficient phosphor layers, whatever works)
Cool running (as in it never rises above ~35C)
Inexpensive (well, for a quality EDC ~50$)