A list of my favorite custom keyboard features thus far, courtesy of TailorKey:

• Caps word: Engages the caps lock function only until you hit a word boundary (anything that's not an alphanumeric character, underscore, or backspace basically). So helpful I'm wondering how I ever typed without it.
• Home row modifiers: Tap asdf or jkl; and they function as normal letters, but hold them and they function as modifiers (ctrl, alt, cmd, shift). Tremendous.
• The "select word" functionality. This is a macro for alt-leftarrow (move cursor to beginning of word) followed by alt-shift-rightarrow (select to end of word boundary). There's also a select line, but I haven't found myself using that much yet.

I've also added some of my own customizations:

• A key that triggers a script that flattens whitespace from selected text
• Keys to uppercase, lowercase, titlecase, and sentencecase selected text
• A key that triggers a macro to type "site:citationneeded.news ", which I type constantly while writing my newsletters and trying to find a past newsletter where I referenced something
  
• A two-key combo to trigger the typically three-key cmd-shift-4 screenshot selected area command
  
• A two-key combo for "paste and match style", which is the hand-breaking four-key ctrl-option-shift-v
• A two-key combo for — (em dash), which is typically three (alt-shift-hyphen)

And I've moved some of my previous shortcuts that tended to bump up against other shortcuts (like alt-arrowkey to snap windows to portions of the screen) to esoteric key combos involving F13+ that will never conflict. (I don't have physical F13+ keys, but by using layers I just map comfortable keys to the shortcuts I want).