markdown is perfect for forum comments, but idiotic to extend too far. if you’re a programmer wanting to write a book in markdown, you’ll find yourself thinking of markdown syntax extensions more than thinking about the book.

if you want extensibility and still a rather lightweight syntax, try restructuredtext. it has exactly one flaw compared to markdown, which is the hideous inline link syntax (which practically forces you to use named links). that makes it less suited for forum comments (where you might want to quickly inline 1-2 links), but perfect for books (where you can neatly specify your link targets below the current section)