Here is my vote (as a xwiki user and admin) :
User Interface
P2P (allow offline usage and P2P XWiki hosting)
GData backend
AJAX API, Toolbar and Form Editor
JCR backend
Editor Completion
AntiSpam features (can't it just be a jcaptcha-like on the user registration
page ?)
Cheers !
And thanks for all xwiki !
--
François Le Droff
http://www.droff.com