I've not read much on AC/DC conversion, on wikipedia there aren't many percentages of energy lost due to ineffiency in conversion. But there are cases where efficiency can be made easily.
I'm thinking mostly of:
1) an UPS that converts AC to DC for a battery, and then to AC, where an electronic product will convert again to DC. What is the inefficiency penalty of converting twice energy?
2) Basically the same but with a house with solar or wind energy. From DC to AC cabling and from AC to DC for electronic products.
In both cases, if only DC energy is used, the energy lost in each conversion is conserved.
To ad to this, as far as I know, AC wastes energy in heat instead of electricity (the "power bricks"). But I think that is also due to AC to DC conversion.
Not to turn this into cornucopian wishful thinking, but on a talk with a friend (...I think he is cornucopian), there was the proposal of using light as it is for energy source. AFAIK, light transmission has only 1 way of distribution, and standarizing on light distribution over fiber as energy source... or using low to medium voltage products that just need light (and have batteries for the night) may be quite nice.