Most of us don’t give our keyboards (or mice, I suppose) a second thought. Most of my keyboards tended to be budget logitechs, which while decent lack a certain something.
I preferred the ThinkPad keyboards on my laptops, but when I need to, and tend to write at a single sitting, essays that are a few thousand words long, I needed something better.
If you want a shorter version of this whole blog post,I’d advice that you look at the layout(ergonomic vs standard, and number of keys), the switch type ( membrane/scissor vs mechanical (switch or buckling spring) and Key style (full sized concave, low profile or flat).
Personally, I’m not a fan of split keyboards, or ergonomic ones. They work for some people, by putting the hands in a more natural position while typing. The split keyboard tends to separate the standard QWERTY keyboard in two (though there’s no reason you cannot use Dvorak or Colemak or your local keyboard layout with them).
Microsoft natural keyboard Pro source
A variant of that theme is the ‘bowl’ keyboard, which splits the keys into two bowl shaped depressions, popularised by the matron and kinesis advantage keyboard. The bowl keyboard design allows for fingers to reach keys with less movement, by fitting around the natural reach of a hand at rest
And finally, the Datahand, which is a pretty extreme case of ergonomic keyboards, with nearly no hand movement. It is very sci-fi looking, and supposedly incredibly comfortable once you get used to it.