Steam is the way of the future
And not just for computers, and not just with Steam itself.
My guess is downloaded content will outstrip store bought games sometime in the next console generation. Motion Controls should earn us a few more years on the current gen before we're seriously hankering for the Playstation 4, XBox 720, or whatever craziness Nintendo comes up with. By then, multi-terabyte drives will be easy peasy, and even Solid State Drives should be up in the half-TB range. Imagine, if you will, keeping a modest library of 50 or so games on your local HDD, with one or two that you're currently playing stored on the SSD. If you own more than 50 games, the others would be associated with your account online, and you can swap those out at anytime.
Getting back to the point at hand, this model allows for things like free games, and content a la carte. If I don't care about CoD's campaign mode, and just want to play Multiplayer, that could be downloaded for a lower price than the full shebang. Or if a new company wants to get their name out there, they could release a couple cheap/free games and a demo of the "big one" to get you hooked. This will not only allow for cheaper games, but it will force companies to put out solid products, or risk being buried under the competition.
This model will also allow for the video game publishers to have direct control over pricing. If Bioware wants to sell Mass Effect 1 and 2 on the cheap (or maybe a package deal) leading up to the release of ME3, they would have to coordinate that with Target, Best Buy, Walmart, etc, each with thousands of physical locations. And they would have NO sway over used-game sales.
We can already see the start of this. From consoles to PCs to smart phones, cheap and easy content is ready to busy out. We just need the infrastructure and devices capable of handling it. Somehow, I don't think the iPhone was designed with 6 straight hours of Angry Birds in mind
Only problem: once content becomes entirely digital, what will I buy friends and family for birthdays and xmas???