I think the biggest obstacle is resale of the house if you go overboard on the automation, especially custom devices.
Agreed, there needs to be a cost-effective system that sits on top of the existing electrical system. Maybe someone has come up with this idea already but add a "Master" controller like an rPI, and allow it to directly connect to a home electrical installation so it can be used as a network bus. Install addressable electrical outlet/light adapters where you want, then program away. If you could run I2C over the distribution outlet/light adapters would just need a small package with a relay and chip. Seems entirely plausible. I'd use Codesys on the rPI for the programming and visualization through my phone/ipad.
If I sell the house, I take it all with me!
Looks like these guys are onto something, although wireless...