Line numbers. Seriously, how does anyone work without line numbers? They add context and reference for conversations and screen shots, especially in playgrounds. Text Editing > Editing > Show, Line Numbers
Trailing space trimming. Tidiness is godliness. Text Editing > Editing > While editing, Automatically trim trailing whitespace. (I’ve never bothered with “Including whitespace-only lines” and vaguely suspect it would mess with indentation levels for braced scoping.)
Keyword highlights. I like the built-in highlighting feature that shows corresponding symbols after a short delay. It’s a subtle cue that emphasizes “where else do I use this?” without having to enter “edit all in scope mode”. Text Editing > Editing > Highlight instances of selected symbol. I use a delay of 0.25 seconds, but I could imagine lengthening that so the highlights don’t appear until I’ve focused longer on a particular symbol.
Spaces. Need I say more? Text Editing > Indentation > Prefer indent using, Spaces. I’m an unabashed 4denterist. My tabs indent “in leading whitespace”. I’m not saying 2denting is bad or wrong but man can it be hard to read and interpret as visual columns. (“Would you like a pamphlet on how 4denting can save your eyes?”)
What about you? What is special about the way you’ve set up Text Editing preferences?