#3 ~ Magnetic tailcaps.
(Don't wear a mechanical wrist-watch if your EDC light has this feature.)
#2 ~ Proprietary batteries.
(Even worse, the ones you can't get at, at all to replace with the same battery from the same company; even if you wanted to!)
#1 ~ Smart flashlights.
(The kind that shut off completely when the light, not YOU, decides it should shut off. And, the batteries still have plenty of juice inside the lights. I've had a couple of these lights over the years. Placed them where they belong.... in the trash!!! Seriously, who was the moronic excuse for an engineer who thought that would be a great idea for a flashlight?)