Features I look for in a headlamp in no particular order of importance:

1) Power source consisting of a single commonly used rechargeable cell. I prefer NiMh AA's.

2) Adjustable output. 3 steps minimum. 100+ lumen high level. Infinitely adjustable would be ideal. The level I need to navigate in the woods is way more than I need for reading in the tent.

3) Light. <2 ounces without cell.

4) Voltage regulation. When the cell is at the end of its charge I don't want the light level to sag. But I also want a warning when the cell is low. Blinking once every 30 seconds maybe. Some regulated lights cut out with no warning once the low voltage threshold is reached.

5) Nice clean output with no rings or artifacts.

6) One handed operation.

7) Cree or Rebel LED. No 5mm LED's

8) Glove friendly switch.

9) Waterproof.

10) Cell can easily be replaced in the dark by feel only.

11) Firm switch that won't be activated inside pack.

12) Simple design with few moving parts. Especially not a pivot hinge.

There is no perfect headlamp IMO because every design is a compromise on features. The light that currently is the best compromise for my particular needs is the Zebralight H50.

A solar powered light sounds good, but given the state of today's solar cells it won't be very small or it will take a long, long time to recharge. And it's only going to work on sunny days, outside of forest canopy and deep canyons. That really limits its use.