I won't store diffs, but it might be nice to see if, when loading a draft, the changes from the draft could be merged into the current edit window, rather than replacing it.
That sounds good.
And another possibility whilst we're kicking around ideas would be to have a "my drafts" link at the top of the page, which when you clicked on it would show a Special page with the list of outstanding drafts, which would make it easy to take up where you left off. Think of "my watchlist" being kind of like an email inbox, and "my drafts" being kind of like a draft emails folder. Oh, and it also it should allow drafts to be deleted, so that you can keep them under control. And it should show the edit summary description, like an email subject line, which would also have the side-benefit of encouraging people to enter useful edit descriptions early because that'd be what would show up in their "my drafts" page. In fact, to make it easier to visualize, here's mock-up screenshot of what I was thinking: http://files.nickj.org/MediaWiki/my-drafts-mockup.png (it's possible to expand this with various sort options - e.g. sort by most recent draft, sort by oldest draft, sort by article name, etc - but you get the idea).
All the best, Nick.