As already recommended, LaTeX for "large" documents: Scientific stuff, letters, job applications, ... When I need graphs/charts/figures, I either use Inkscape to create SVGs or tikz.

For "smaller" documents or slides, groff is nice, too. Especially grotty, the driver for terminals, comes in handy when you need to output "plain text". I also use groff to write those short notes in my gopher hole ("twitpher").

Markdown + pandoc sounds intriguing. I've only recently started to use Markdown on a regular basis.

Of course, none of those options are WYSIWYG. Are you looking for something like that? If I remember correctly, MS Word for DOS was ... kind of ... like ... WYSIWYG. I've never seen something like that in the modern Linux/BSD world.