FCKEditor

FCKeditor extension enables a more intuitive WYSIWYG editor when editing pages on a MediaWiki-based site. It uses a special version of the FCKeditor, a WYSIWYG editor that outputs wiki text rather than the usual HTML that caused problems for MediaWiki integrations in the past.

Issues
There have been enough issues with FCKeditor v1.0.1 (particularly in conjunction with MW 1.16 and SMW) that it is not a recommended version to use.

MediaWiki 1.16
FCKeditor v1.0.1 will not appear in MW 1.16. A patch has been made on r5859, but has not resolved every PHP error.

EDIT: as of 1/10/2011, there is a fork of the original FCKeditor extension that is available for MW 1.16

SMW wikitext syntax
FCKeditor will change or not covert SMW property definitions into links. A change to the SMW code is required (for SMW 1.4.1).

SMW templates
If the FCKeditor is enabled and not completed loading upon saving a page, all templates referenced on that page will be broken. FCKEditor will append "fckLR" at various points or replace it entirely with an HTML tag reference, breaking all reference to the template. This has been noted in MW 1.15+. This is a definite deal-breaker.

tags
On pages using  tags, FCKeditor will add or move a inappropriately and add "fckLR" text to the last line of the next. The onus is on the user to change the incorrect text upon each edit. This has been noted in MW 1.15+.

Loops extension
FCKeditor may not work appropriately with Loops editor. It gives PHP run-time warning notifications that "Object of class ExtLoops could not be converted to int". This has not been confirmed. This has been noted in MW 1.16.