When editing a page, it would help to have a "preview changes" or "show diff" button, which brings up a diff from the current page content to the proposed new page content. --JoshTriplett

Some discussion from the main discussion page:

It would be nice to be able to have a button to show "Differences" (or "Show Diff") when editing a page. Is that an option that can be enabled?

It's doable, it could even be done by a plugin, I think. --Joey

See the editdiff plugin --JeremieKoenig