Brighter - sometimes important and sometimes not. I've got my Barbolight u-15 if I want bright, Infinity if I want dim, and I could grab a variety of lights for dual stage - A2 Aviator, 27LT-S, PD, Eternalight, and it goes on and on..
Longer runtime - subjective. I've got a 4D Maglite with a cheepie dropin with 4 3mm LEDs. It'll run for weeks. Realistically, how much runtime do you really need?? Longer is better, but it's at an expense.
Reliability - I don't expect failure from any of my lights, shy of having a battery go bad or something. What I DO expect is customer service, so that when my light dies I can get it fixed. Surefire does it at no cost, and I've never had problems with Inova for example.
AAs are good, don't get me wrong, but for my uses lithium cells are better. They're better in hot and cold, they're lighter, they have a longer shelf life, they put out more for their size, so on and so forth. I've got all kinds of light that take all kinds of batteries, AA, Li-ion, C, D, even 9V. I don't discriminate. But when I need to trust a light, it has a lithium battery.
Cost - important, don't get me wrong, but not the end-all be-all. Otherwise you'd see everyone in America driving a sub-$10000 kia and not a $400000 SUV..
Ergonomics are very important to me. I carry a flashlight at least 12 hours a day, and I want something that's pleasant. My wife has a Fenix in her purse, as I just couldn't get used to it and didn't care for the feeling. She likes it a lot, but I don't. Personal preference.
These are just my opinions, btw. And one of my favorite little pocketlights is my Nuwai QIII on a 3.7v rechargeable battery. Not my majority EDC light, but it finds its way out every now and then. I guess I just figure if I'm going to use something, why not get the best that I can??