I've long wanted to run DC in my house, but there's no standard. Do I run 120v, 240v, 5v, 12v? Which interface do I use? Barrel connectors, USB, cigarette lighter? If we have a large-scale movement to household DC, we'd have these things hammered out without me rewiring every appliance.

If these obstacles were overcome, almost everything in my house could run on DC. Most stuff either converts to DC internally, or doesn't care.