Well, one potential issue that springs to mind is that DC 120V might overstress the diodes in the full bridge rectifier. With 120V AC only 2 of the 4 diodes are active at any one time and on average each of them is on only half the time - with DC only 2 of the diodes would be active, but they would be active 100% of time. Say the power supply needs to draw 1A of current, a reasonable good estimate for the voltage drop of a forward biased diode is 0.7V, so each diode that is on will dissipate 1A*0.7V = 0.7W. With AC, each diode would only be on half the time, meaning it would dissipate an average of 0.35W. With DC, 2 of the 4 diodes would be on constantly dissipating the full 0.7W each - this will result in the two diodes getting much hotter. Ask yourself how likely it is that most power supply manufacturers are designing in a safety margin of *2X*...

And that's just dealing with power supplies for electronics (which mostly just want low voltage DC) - if you get into appliances, while almost all will have a power supply that converts to DC, they usually have some circuits that use AC directly (think heating elements, safety critical sensors that use the 60Hz AC waveform as part of their feedback to make sure they haven't locked up, etc). Even appliances that convert the AC input straight to DC (think anything using an inverter to drive a motor or compressor) will often use a voltage doubler circuit to get a DC bus that is ~340V from the 120V AC input (it doubles the peak voltage, which is actually ~170V and not 120V in US) ... The voltage doubler circuit will not infact double a DC input, and the appliance will likely be most unhappy when it's DC bus is a lot lower than it was designed for.