Oddmuse has a very simple core: A single Perl script, barely any formatting rules. It has wiki words, free links, and other kinds of links, and bullet lists – no emphasis, numbered lists, tables, etc. Everything else is added by dropping extensions into a particular directory.
The core is very small (less than 4000 lines of Perl), uses flat files to store data (no database), and is easy to install (a single file). Those are the most important design constraints.