Choose wired or wireless.
If wired, choose USB or PS/2. (Note that PS/2 offers no real advantage anymore, now that full NKRO emulation on USB is available, unless you need a PS/2 keyboard for legacy motherboards and legacy BIOS.)
If USB, then USB2 (even USB1.x) is good enough, unless the keyboard has embedded USB3 hub/ports.
Choose keyswitches. Cherry MX mechanicals are widely acclaimed as the "best". Some people prefer ALPS, Topre, or one of the growing variety of Cherry clones/variants (Kailh, Razer, Romer-G, QS1, Novatouch, etc). Specific switch type really only matters if you're a typing elitist, although some Cherry flavours are better than others for keyboard-mash gaming, and some Cherry flavours are louder or quieter (some typists like audio feedback on keypresses, most people around typists don't).
You'll encounter many references online which condemn non-mechanical keyswitches. I do agree that "membrane" switches are simply awful, they feel "mushy" to me. I disagree with the consensus of "rubber polydome" switches always falling into this category - there's different types - I find that many do indeed feel unpleasant to type on, but many actually feel very crisp and pleasant, in rare instances good domes actually feel better than bad mechanicals.
A secondary aspect to keyswitches is keycaps. They're often interchangeable, and you can customize your keycaps countless ways. Usually (though not always) a keyboard with standard Cherry MX switches will also use standard Cherry MX caps. Much less of an option with keyboards using "nonstandard" switches.
Backlighting is great for dimly lit conditions. Easier to use the keyboard in the dark. RGB rainbows are just aesthetic.
The usual bunch of multimedia keys are always useful. Headphone/mic audio jacks (and USB ports) on the keyboard are awesomely convenient.
Programmable macro keys are useful if you use them. Some games, productivity apps, and code/dev environments make excellent use of these timesavers. On the flipside, having too many keys can become a bit of a nuisance.
"Tenkeyless" and other form factors which chop off the Numpad and shrink keyboard dimensions are loved by some, especially those with Hipster style preferences or limited desk space. I personally don't see the appeal, they're too truncated for me to find them at all functional.
Built-in display screens (or docking ports to let your smartphone function as one) are a gimmick. I've actually tried to *make* these things useful but in the end I've found they just never are. Maybe more useful for people who don't run multi-monitor setups, lol.
People unfamiliar with keyboard switches/etc can learn a lot about their personal preferences by simply going to any store which has a bunch of keyboards on display and spending a couple of minutes typing/playing on each one.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams
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