Sorry, I simply meant to highlight that the voltage conversion process itself uses some power, not that DC is less efficient. Efficiency depends on the individual device design & usage regardless whether it's designed for AC or DC. Even the conversion process from DC to AC, using an inverter, loses some efficiency. The inverter robs some power to do its job. While the 12VDC furnace fan you mention may be more efficient, you still need a device (transformer/rectifier) to bring your 120VAC down to the 12VDC. The transformer/rectifier uses some power itself, not much mind you, but if you had a 12VDC supply in your home home & could ditch the TRU, you would run even more efficiently, saving additional power over the long term.
Ideally the grid would provide us with both AC & DC, this way we could choose to run either type of device without converting the voltage using drivers, xformers, rectifiers, power supplies, etc. in our home. Not that far fetched these days even. The province of Manitoba has the distinction of having some of the longest DC power transmission lines in the world. Trouble is, they convert it back into AC before connecting it to the grid since this has been the standard in homes since the dawn of the electrical age. People with solar/wind/micro-hydro power that are also still connected to the grid already have this option to an extent, depending on the capacity of their home power genreation system. This is why I'm thinking of getting a solar array & some batteries. I already have a small wind generator, but haven't set it up yet. Once set up I can run DC devices such as my VorTechs from the battery bank using 24 VDC with 12 VDC as backup and also a certain amount of my LED lighting straight from the DC source. Come to think of it, I believe the MR16 LED lamps I have already handle either 12VAC or 12VDC. An old MR16 LED I salvaged out of a fixture was polarity sensitive, I had to plug it into the socket the correct way, my new ones automatically sense, so they're foolproof.
Here's a link to a pretty good article about the development & usage of AC LEDs. A little technical in parts, but plenty of less complex info as well.
http://www.ledsmagazine.com/features/3/5/2