It is sad that the best editors that come native with wikis are in non-opensource products. eg. Jotspot (which has just been bought by Google), Seedwiki and Wikispaces for instance. It’s to do with markup language.
Mediawiki is probably the premiere OS wiki. There are several efforts to get a wysiwyg editor to work with it. One is wikiwyg. Requires a specific script (greasemonkey) which at the moment I cannot get to work. There are also some notes on the Mediawiki support wiki about the issues around editors. I guess the developers of Mediawiki just don’t want to develop and maintain an editor. I hope something emerges where we can plug in an editor without complexity. I guess I am aware of the main problem: tables and images. I’d be interested in hearing from anyone with a solution going.