Wow! This is some serious work, and extremely useful. Thank you very much. Btw, in the Creole plugin for MediaWiki I use <<...>> for macros -- it's ok as far as I understand the definition of "placeholder".
Update: I have lokked at the markups of the colliding wikis, and I noticed one more thing that is not mentioned on WikiMatrix, yet often creates a conflict: Many wikis use the "#" character for comments and/or various pragma directives or macros.
-- RadomirDopieralski, 2006-12-29