Initially, I stayed away from CR123a batteries and lights that used them. The relatively high cost and seemingly low capacity for that cost was enough for me to stay away. The long shelf life compared to alkaline batteries was the reason that I started using them.
Alkaline batteries: easy to find and relatively cheap, especially at Costco. Leakage has caused me to switch to LSD NIMH batteries. Non-LSD NIMH batteries seemed to only last a year or so mostly because I'd let them sit, unused, for months and they'd drop to below 0.7V. I now use my non-LSD batteries as much as possible, recharging them constantly, so they have been lasting. My LSD batteries are charged once every three months to keep them from failing.
QTC: I really thought that QTCs would be the holy grail for me. You see, I really like multi-mode lights and don't really like single-mode lights. The QTC allows me to, if I could figure out a way to incorporate them in a light, to vary the output of my single-mode lights, thereby resolving the issue of what to do with them. What I've found is that they're somewhat fragile and will fall apart if crushed with too much pressure or if a twisting motion was used while twisting. In addition to that, if used in conjunction with a driver, you'd get what I call QTC drift: set your level then the light would slowly, on its own, would brighten. Readjust it back down and it would settle again, though, this time to a lesser degree. On lights that are direct drive that use QTCs this also happens, but, to a much smaller degree and so it's not as noticeable.
Lights that use a single 18650: for me and the way I use my lights, lights that use a single 18650 just don't seem to work for me (and my collection of lights is dominated by 1X 18650 lights) because they're too big for pocket carry comfortably, which means that they're carried either in hand or in a backpack. At that point a 2X 18650 light isn't that much bigger, but, they're brighter and/or longer lasting than a 1X18650 light. Yes, I could carry one in my jacket pocket, but, it's on the large side for this purpose. AA and AAA lights are easily pocketable and you'd forget they're there. That light that does everything perfectly for every situation is worthless to you if it's sitting at home because it is too big to carry with you when you need it. If you're at home, unless there's a power outage, you don't really need a flashlight most of the time. This also applies to lights that use a single CR123a due to the wider width, though, these lights do fit my jacket pockets much better and are my go-to lights when I think a AA light might not be enough but I want to travel light.
Turbo head lights: throw is good. Throw is bad. Throw allows you to see things far away. I'm slightly far sighted in my left eye and 20/20 in my right eye. I understand the allure of throwers as I can identify objects at 2-300 yards if I can put enough light on the subject. My cousin, on the other hand, is near sighted and even with correctional lenses he can't see well past 100 yards. To him a thrower is useless. A thrower close in gives you a tunnel effect so it's not the best tool for this purpose, which makes them more of a specialized tool.
Mules: I've never appreciated mules until recently. I've actually taken this concept a step further by having lights or frosted diffusers that project light both forward and backwards, leaving only a small cone of area directly opposite the emitter where there isn't light projected. Most of the time that I use flashlights it's indoors and close in. A mule will cast light on a larger area and the lack of a hotspot to blind me is really appreciated.
