Have you looked at Zope's structured text? They have 3 general formats for text input: Plain text, HTML and structured text.
Structured text uses text indention for text structuring and has shorthands for making links, bullets and various text formatting.
Also their wiki looks pretty good.
(See http://www.zope.org/Documentation/Articles/STX or http://www.zope.org and http://www.plone.org for examples of its use.)
