In particular, this announcement opens the doors for independent casual and episodic game distribution through one central hub. Gaining access to the long tail of PC gaming is a great move for Valve, especially in competition with consoles. While blockbuster games, being graphic-intensive, will likely do better on the console than the PC market (due to the cost difference between a PC capable of playing the game well versus a console), independent games often have lower technology requirements, focusing on innovative game mechanics or story concepts to pick up interest.
By hosting a bevy of compelling "hardcore" indie games, Steam will likely do very well in increasing its audience. And considering the recently added social networking features to the service, once gamers come to Steam for content that can't be found elsewhere, there is a better chance they will stay and peruse the other offerings.
Things are steaming up indeed.