These threads are always very strange to me.
Rather than fabricate a list of arbitrary criteria from a position of ignorance, I relied on a process of empirical experience.
Beginning with give-away/throw-away keyboards, I kept my eyes open and snagged any keyboard that tickled my fancy.
After actually using dozens/hundreds of keyboards under my fingers, over months/years, I gradually developed my preferences.
I understand your feeling.
I'm always suprised when some friends ask me which perfect mouse they should buy to fit their need.
I own dozen of them and still looking for The Mouse. I tell them to start with some popular mice to find out their grip, weight, sensor, etc.
But beside my QWERTY Atari 800XL keyboard, i never owned a mech keyboard.
[whining on]
I would like being able to get some keyboards, test them after some months, etc...
But i don't have the cash to do that: importing new ISO mechs from US/UK is way too much expensive (shipping cost + VAT).
On top of that, in France, there isn't any shop where you can test mech keyboards (well, in fact some shops have the Logitech ones...).
Friends don't have any mech keyboards (they all have rubber domes) and most of the mech keyboards on ebay are either Logitech, or don't have a french layout.
Maybe i'm too old and only young gamers have mech keyboards.
Of course i could swap to ANSI/ISO-UK to have more choice, but i will still have to import them.
I dont have this problem with mouses because most of the brands (Zowie, Mionix, Roccat, SS, EVGA, etc.) are available in french retailer (Amazon, LDLC, etc.).
The arbitrary criteria you are talking about are new to me because initially, i wanted ONLY a backlit keyboard and being able to customize some color.
THEN, while browsing popular site like WASD, MAX (etc), they asked me "which switches, with O-Ring or not, which layout ANSI/ISO (FR, UK, DE...), which profil, etc, etc, etc...
I did not thought i will have to choose all those things at first for a backlit+some custom keycaps!
This explain why i (we, beginner) need to do those choices "arbitrarily".
And i can't test anywhere to find out: no availability in local shop, even online (ebay), no friend having them, expensive shipping cost, layout problems make it even more worse.
If only those keyboards were way much popular...
[/whining]
TL;DR I go back to my old computer keyboards