It seems I was not quick enough but I will place in my take but some information may be duplicated from others. And also a lot of stuff with keyboards are personal preference and no right or wrong so the odd thing I mention may be biased to my preference of the keyboards I use.
For me the most important part of a mechanical keyboard is the keycaps, I would look for PBT if you can as this is the type of plastic that will not shine through use and is a lot more durable. A lot of keycaps are ABS which is the type of plastic that will shine with use. There is also keycap thickness, which makes a difference. That being said there are good quality ABS keycaps, but they will still be susceptible to plastic shine.
In terms of numbers and letters usually the best types are double shot and injection mold, this will mean the characters will never wear off with use.
For switches there are different weights before they actuate, which is the force required to push the switch down. I think my ones (Browns) are 55g from what I remember. There is also the travel distance which is how far you have to press down to activate the switch, so it’s a combination of weight and distance.
The other thing is depending on the keyboard the switches either go straight to the PCB and that’s it or some keyboards will have a metal plate between the switches and the PCB, my preference is a metal plate as the keybaord is more solid, won’t flex as much
I have had clicky and tactile, at least with cherry switches I have had blues and browns. I started off with blues and switched to brows as these were just way too noisy for me and just like the bump feel.
One other thing is I don’t know which country you are based but at least for me I bought an American ANSI keyboard while I live int he UK. The reason being that if you want to buy aftermarket key caps, there is far more variety for American layouts and cheaper that European profile sets which are a lot more uncommon.
In terms of Size there are many as Dalto mentioned but I have an 80% as I hate the number pad, without the number pad I can have the keyboard centered to me when I am typing and out of the way of the mouse.
If you are curious of the keyboard I have It is a Filco Majistouch 2, however the key caps they provide are ABS and a bit cheap feeling, I already had aftermarket key caps so this wasn’t too much of an issue.
I will add another comment if I think of anything else but this is what comes to mind so far.