There are some decent open source solutions, or you could provide an upgrade/add-on that we could pay for if you chose a commercially licensed editor. One I really like is XStandard but licensing is a bit expensive. There are a ton of others though, including TinyMCE and FCKEditor.
I know not everyone wants an HTML editor but I'd kill to have a SIMPLE one (bold, bulleted lists, center, insert image tag, etc) and if it was integrated with the ShopSite media directory it would be even better. My clients' number one complaint is having to learn basic HTML.
I think it should be an option in store prefs... use WYSIWYG or not. If checked, the textarea blocks for most fields (like product description, page text, product fields, etc) would be WYSIWYG fields.
Thoughts?