Valve is introducing a fancy new content system to Steam that will make your life less miserable. For the consumers, the new sytem improves bandwidth and uses HTTP for a more firewall-friendly solution, and Steam will be better equipped to handle traffic spikes during things like hot new releases and Steam sales.
It also means updates and patches will only require you to download the new data instead of having to download the entire updated game file from scratch. That's really good news if you have The Witcher 2 on Steam.
For the developers and publishers the new system simplifies the process of publishing and updating their games on Steam, and this way the digital distribution service will be better equipped to "ship" more games than was previously possible. But that's not all.
Valve states that the new content system will also allow them to build new features that have been requested for ages now. Future updates to Steam will finally allow you to download an update while playing that game(it will update the game once you exit), and they'll be working on features like download scheduling, bandwidth throttling, and prioritizing which games you want to download first so you can actually play them.
Hurray! Hopefully all of these new features will be place before the next Steam sale.