Ummmm. Valve needs to be a lot more clear on this.
correct me if I'm wrong, but this doesn't seem that big. Isn't this essentially what the Xbox One should be able to do at launch. One console, anyone can sign in as their own account, one Gold membership, everyone can access the same games. Hell, isn't it just the equivalent of what you can do on 360/PS3. Multiple accounts, one system. Everyone gets their own saves and achievements?
Now, if it is in fact multiple computers which can access your library, than this sounds like it's the same as if the games were on disc and you can lend them around to your buddies. Only one person at a time type of thing. Valve seems to have solved what Microsoft was trying with Family plan thing they had. Their problem was mixed messages and DRM for consoles which is hard to sell. Now for Steam, it's easier. They've been around long enough. Taken enough shit early on and are now a trusted business. I guess in that sense this feature can be big.