Very interesting! :-)
I'm a beginner in js stuff, but I realized how much js can be useful in a number of ways (presently I am a "very simple gadgets maker" t help users engaged into proofreading into wikisource). I'm a little worried about additional server load coming from AJAX, and I suggest different policies between local js scripts (that, fix me if I'm wrong, don't imply any server load) and AJAX scripts, which raise performance issues.
So, I can't wait for a js wiki.... but please, keep it simple! So simple that a beginner too could browse it,learn from it and publish anything without any embarrassment. Beginners often feel themselves really stupid.... and some good idea could be lost.
Alex brollo