There's probably some kind of "social network graph" hack that could be applied. Like you could share any game from a family member... but only one singular family member, and you can only change your singular family member "link" on the 1st of January or perhaps the anniversary of when you signed up with Steam. That seems fair.

So my HTPC and my kids desktops can each sign up to share Dad's personal library... but they're both stuck to me, and me only, for a year. In a way I kind of like the idea of my kids not being able to "borrow" some ridiculous 17+ rated gorefest from the other side of the internet or a kid at school... I know they're stuck on my account, or at least I have a vague idea what they're doing.

How would you enforce people not creating accounts merely to share one dudes library? Could make it terribly slow or inconvenient perhaps?

Another interesting option is creating tiered accounts to prevent widespread pirating-type operations. So you can either be a sharer or a borrower and never switch between. Or as per above, switch as many times as you want, once per year.

I'm sure there's some moronic and expensive patents out there to prevent the whole thing from ever getting off the ground.