Actually there's something similar that exists: docweb (See http:// doc.java.sun.com/DocWeb/)
Of course it would be a killer app if we could make this in xwiki. The killer feature would be of course that after review the changes are incorporated in SVN (using svnkit for example).
If by "killer feature" you mean the one that killed the idea, I'll agree... We can gain a great deal of benefit (including support time not stolen from developers) if we can get something going and start collecting content and helping keep users off our backs; worrying about how to incorporate the content into the code can wait for when we have some content to incorporate. If you have to cut and paste the content by hand it's still loads quicker than having to make it up yourself... And besides, it's the Wiki Way...
Thanks -Vincent