Screenshots are nice but what Steam Cloud could really use is a saved games abstraction.
Like, there's only a few games which use the Steam Cloud for saved games. But it would be neat if Steam could retrofit it for all their games.
All of the games on Steam that have saved games keep their info in some hard drive location. It wouldn't be too terribly difficult to go through and find out where all those places are and then have Steam upload them to a server, presumably while Steam is idle and not playing a game. Same for the config settings.
That way, not only could you have Steam go in and re-download all your games, it could also re-download all your configuration settings and saved games for all of your games and you'd be set.