For me, org-mode is my go to markup language. So much so that I now find writing in Markdown to be almost painful. A few of my favourite features are:
- Tables are amazingly easy in org-mode, with spreadsheet features thrown in as well.
- Code snippets can be written in their own major mode, giving me syntax highlighting, auto completion and the ability to run the code and embed the output into the markup.
- Local links autocomplete, so I don't worry about breaking them with typos.
- I can export to HTML, Markdown, ODF or PDF.
- GitHub supports org-mode Readmes
For me, babel is the killer feature. I can write code snippets in the buffer, highlighted and indented according to the corresponding emacs mode, with paredit for lisp, and beautifully exported via pygments. A single key combo lets me evaluate my code in persistent repls associated with my buffer, and I can choose what combo of code/result I want to export. I had some of my snippets generate raw LaTeX that I could include in the document.
Org links make section references trivial, and I have it hooked up to ebib so my citations are pure org. In the emacs buffer, clicking a citation link takes me straight to the bibtex entry.
I don't much use the organiser, but I do use org heavily for writing cross referenced notes about other people's code.
I actually, currently, use Org-Mode much more for writing than for organising / tasks / todos.