Glued lights, they are hard to open as I mod almos all my lights.
First and foremost: PWM. If I can spend 30 bucks on a light that has a linear driver, why would a similarly priced Maglite that is available everywhere (MAG certainly has more working capital than XENO) still using a PWM driver. It's not even high frequency, it's like 75Hz.
Second: Cool white LEDs. The difference in output between neutral white and cool white is so minute in the latest LEDs that you really won't notice a difference in output. Even if you do, the sacrifice is worth the "everything doesn't look dead to me" light.
Same.
1. The generic oval machined cut-out on the body with the brand name (this isn't the first time I vented this frustration, haha)
2. A tail cap/switch that is so close to allowing tail stand but still can't! 
UI
Any flashlight with slow/sloppy mode retention feels instantly cheap to me. UI is extremely important because it is how I interact with the device. Just like the touch-screen on my Droid, the crucial connection between me and flashlight must be seamless and should approach being a natural extension of my hand. Nothing I hate more than a torch doing something I did not expect or want.
machine quality
Call it what you will, metal burrs and road-to-Gibraltar lathe work turn me off the second I open the envelope. Also, painting a flashlight black and calling it "anodization."
And, I guess my torch taste has yet to mature because I like different colored boots and GITD lens o-rings. The boot is usually the first thing I change upon receiving a light. I'll probably grow out of it one day.
Foy
Stupid drivers like: H > M > L > SOS > Strobe
I would rather prefer Low (the lowest the better) > Turbo (the brightest the better, with at least 30min of runtime) > M (5h runtime) and may be other hidden modes (like strobe by clicking quickly 3 times, or keeping it pressed), or beacon (by cycling all the modes a couple of times).
I also HATE slow PWM on low modes, it even gives my headaches.
Strobe!
It's funny. Years ago all (most) flashlights cheap and expensive had a side switch because that's where your thumb naturally goes. Now almost every light requires you to carry it like a spear to operate it. I'd like to see more well made side switches.
I hate that they need batteries
Because I like to randomly blast things for short periods at a time and frequently, it bugs me when the mode memory takes a long time.
Couldn't agree more! Exactly what I meant in the OP about protruding switches but I phrased it badly. Never designed to tail stand is fair enough, near but not quite should be against the law..
I agree with almost all of these complaints
i'm with OXy there is no real deal breaker I'm a diehard flashaholic ..
BUT these make me crazy
#1 - PWM
#2 - Strobes
#3 - Foul tint
honorable mention ..
no low ...
bad sequence I like low med high ....
agree with weiser ...destroy a lights feel so you have a place to put your name
Ringy ,bad beam
stupid spacing 10 lumen /30 lumen 100lumen 200 lumen ... looks like low ,low ,high ,high.
hollow pills , horrible cheap parts
Glued bodies was a good one