I've recently spent a bit of time playing with the YUI editor, which was incredibly feature rich and I liked it a lot, but seemed a little bit heavy for my needs. Obviously it's not right to compare the YUI editor (which is full WYSIWYG) to WMD (which requires a little more tech savvy user base), but the lightweight size and clean styling of WMD is really appealing right now. I'll try and write a bit more when I've thrown it into an app and had a play with it.
What's really worrying me more is that out of all the editors I've seen recently, the one which impressed me most was jsvi - I need help. Seriously.