Having when editing 50% of the screen place occupied by the wiki
source and 50% occupied by the preview could be cool.
Especially if the preview is in a scrollable box, scrolled by default
to the position of the cursor in the textarea and updated in realtime
(This have to be optional since for slow computers it could be
dangerous :D)
Even if the preview isn't perfect the fact that it could be "real
time" and on the client side make it usefull i think.