Updates are getting tuned up in a big way -- instead of re-downloading an entire file to install one update, the new system will only download the changes made to the file, because that's what an update means. It will also be possible to update to a game while playing it.
Valve has written new tools for developers and publishers to simplify their behind-the-scenes processes and ship products faster, promising that publisher-side updates will also be streamlined, which is something EA in particular has been quietly vocal about changing. The new system will be applied to "more and more" content over time, beginning "soon" with the release of Dota 2 later this year. You can try it out now by downloading an HD trailer from the Steam Store.