I can't believe no one has mentioned UI yet. Is your father a 'gadget guy' that will be interested in learning a complicated UI? If he's anything like my dad, giving him a light with a complicated UI will just get it thrown at you the next time you see him... :laughing:
My dad is quite happy with a single-mode Deerelight dropin for his G2. He even figured out the EX10 I got him for backpacking--he just mashes the button until it's as bright as he needs. Anything more complicated than that earns me rants about how computers are destroying the world...
For a general-purpose light, I would recommend sticking with something simple, like the EagleTac-style 2-mode interface (head tight for max, head loose for "general").
For batteries, I agree with Marduke. Unprotected Li-ions? VERY BAD idea. They
probably won't go off like a grenade*, but they will probably end up as single-use batteries when the charger refuses to charge them (because they are over-discharged), and then what's the point?
Personally, my theory is that CR123As should actually be the
non-flashaholic battery of choice. If you give a AA flashlight to a "normal" person, they are going to stick alkalines in it. That means that the batteries will rapidly drop out of regulation, leaving a dim, mostly worthless light that will stop working as soon as one or more of the batteries leaks and destroys it. CR123As, on the other hand, deal much better with high current, making the light more useable for longer on a set of batteries, and won't leak as they're going dead. They are also simpler than rechargeables, because you don't actually have to charge anything (which 99% of normal people don't want to deal with). The only problem is the expense, but you can get packs of decent CR123As for about $1.00 / battery online. For those prices, even
my dad figured out how to order them online.
If you absolutely have to stick with AAs, then go with 2xAA, as 2 alkalines can stay in regulation much longer than 1. I recommend looking at selfbuilt's
2xAA round-up review to find something that does well on alkalines.
* But, then again, they might.