There are several features you could want or ignore in a head torch. So this answer is subjective.
A good head torch should be bright of course with a good distance to it, you don't just want to be able to see a few feet in front of you.
In other situations, you want a wide illuminated angle, so it would be nice to be able to toggle between this mode, and a powerful spotlight.
Furthermore, you might want to be able to use red light to maintain your night vision.
If you could adjust the brightness for each of this operation modes, very good! You don't need to be able to light up a landing strip, when you just look for some socks in your tent.
You want your head torch to be somewhat water resistant or water proof, to be able to wear it in wet conditions.
Good battery life can be really important when it comes to extended use! Same goes for a comfortable, adjustable head band, and your torch should be lightweight, and snug to your head.
All these things are important, however there are some nice quality-of-life features, or gimmicks you could look into:
My head torch allows me to lock it by holding the power button for some seconds. To unlock it, I have to press it for some seconds again. No more empty batteries after your torch lit up the inside of your backpack for 20 hours straight! It even has a battery life indicator - very useful!
If the angle of the lamp is adjustable, you can more easily do stuff in front of your face, or the normal height, when you are tying knots, or do anything that requires your light to not shine at something at 90 degrees of your forehead. I never realized how much I love this feature, until I didn't have to temporarily move my crappy old head torch down to my neck to do something in arms reach below nose level.
For a more "serious" use, you might want to have a strobe function to defend yourself, or signal SOS.