In wiki pages with dynamic content (e.g. http://wiki.dandascalescu.com/howtos/catalyst/introduction_to_catalyst.edit), it may become annoying for said content to keep refreshing as the user types.
A toggle for live preview, on by default, would be nice. Ideal level of persistency would be per page because some pages are dynamic-content heavy and others are not.
Per-user persistency (one user thinks the page is "heavy", another one not) would be overkill. Besides, the "heaviness" can be thought of as an attribute of the wiki, as a sentence saying "This is is a heavy page", which the other user deletes.