A few facts that, for some reason, haven't been stated in this discussion:
1. EMI/RFI--AC systems radiate a lot of noise. Some come from lines discharging through a natural diode and the rest is the lower frequency 50/60 cycle AC and its harmonics. Even discounting the EMI-sensitive people out there (whether you believe them or not), there's no question that it affects receivers and some other electronics, and occasionally magnetics like a credit card. DC normally shouldn't have this problem.
2. Outside of motors and other heavy loads, most loads outside of factories and such are light loads, thus 48-60 volts with light gauge wiring might be sufficient for short (household or small business) distances.
3. A *BIG* safety issue no one has talked about is the increasing danger with voltage and going DC. This is a 2 pronged problem:
A. with AC the voltage goes down to zero every half-cycle, which is why switches can be made relatively simple under that 400 volts mentioned. But this disappears at DC and an arc or spark that fires WILL *STAY* FIRED as long as the power remains applied! I learned this at a GM discussion for mechanics when they were discussing the (new then) Chevy EV1 and its competition from Honda and Toyota when they talked about why they chose 36 volts for that car: its the highest voltage that won't carry a stable arc. The speaker pointed out that a pin hole, which would self-heal and not normally cause a problem on normal 12 volt circuit, would cause a self-maintaining arc at 48 volts if the hole came into contact with the frame. Of course, we all know what happened: the car failed due to lack of range (using lead acid!) and power (ohmic loss). But the safety issues remain, even on modern cars with high voltage DC battery packs, even though they seem to be solved...
B. Above 400 volts or so flash danger becomes a real threat, as lots of Youtube videos attest. Going to DC this could be a lot worse, I would think, as the arcs wouldn't self-extinguish. Combine this with heating inside the cells due to internal resistance and you could see a big BOOM!