What I really want to see is an 'opt-in' service that allows you to use other peoples' networks if you opt-in to sharing your's. It should be extremely easy to opt-in, and a message should be displayed asking you to opt-in when you try to sign in to someone else's network.
Also, the guest network traffic needs to be sent over a vpn, so you are not responsible for it / don't have to deal with people torrenting, etc on your network. And it should be low priority traffic.
That's the problem they need to solve, opt-in and making it easy and visible enough to get enough people to enable it to make it actually useful.
A VPN or similar system to make sure you're not getting accused of things their free wifi is being used for too is a good idea. Not sure if anyone has done that yet in the various implementations.
Also, the guest network traffic needs to be sent over a vpn, so you are not responsible for it / don't have to deal with people torrenting, etc on your network. And it should be low priority traffic.