Your "main" torch needs to be indestructable. made of metal,Have TWO o ring seals,Have battery sizes that are readilly available,Have a slide type on /off switch,Be LED,Have a Pencil type beam,be small enough to go in a BC pocket.
No torch needs to be made of metal, and it's probably better if it's not. Derlin never corrodes, and cannot be bent or dented.
While two or more o-ring seals are ideal, you can get away with one (witness the H Scout and Mini-Explorer, both of which are frequently panned for being overpriced and underbuilt, but which have single o-ring seals and are generally good, reliable lights).
Having a slide-type switch is also not a necessary thing, and is probably a bug rather than a feature. Twist on/off is simple, reliable, and eliminates another failure point caused by adding the switch. If you need a switch on a primary light, the best ones tend to use booted flip switches.
LED is probably better than HID for recreational use: it's cheaper and lot more abuse-tollerant. At the same time, signaling type beams (pencil thin) are not great for rec diving because they do such a poor job of lighting up the pretty corals rec divers are generally trying to look at. Thin, punchy beams are more the province of overhead/tech diving, where light communication is more important than just being able to see pretty stuff better.
Lastly, the 'small enough to fit in a BC pocket' thing depends on what you're diving. If you have a BCD with no shoulder d-rings and aren't diving with suit/shorts mounted pockets, I suppose that matters. Putting a 3-C cell light on your chest harness for a BP/W rig is just fine, however, and in a pinch the same length light fits in most dive suit/shorts pockets.
