- reduces cost of living (cheaper than car ownership)
- reduces number one cause of human mortality
- frees up tons of parking-lot space for additional housing
- reduces strain on roads offsetting transit maintenance costs
- reduces pollution
Focusing on optimizing and improving quality of transit rather than just having it for the sake of it makes a huge difference in the quality of life of the area. This can be done in non-traditional ways by having a fleet of transit vehicles that operate like UberPool instead of along fixed routes, etc.
The other space I'm thinking of is a lot harder for startups to attack, but is worth thinking about anyway -- international labor mobility. This requires world-wide cooperation to work, but there are countries like Japan and Sweden with declining population but still fairly difficult immigration procedures. The world's resources would be used so much more efficiently if it were easier to redistribute people based on their skills, needs and desires with places that had the space and resources to support them and benefit from them. Technology can certainly help here, but it needs a lot more logistical ingenuity than technology.
Another one is reclaiming human resources -- prisoners and the destitute. Majority of these people are able-bodied and/or able-minded, yet it is nearly impossible to gain any value from them because of how society is structured around their status. If we had dedicated, streamlined processes to appropriately assess and reinstate these people back in society, they'd turn from a economic burden into an asset.