In fact, USB has several issues : it is 5V while almost nothing works with 5V right now, which implies a conversion everywhere. But if we want to put converters everywhere, 5V is very low to start with, it'd be better to start with 18-19V like most notebook adapters. Also, the USB plug delivers low power which is often not enough to power a hard disk, reason why most USB/ATA adapters ship with a second wire to double the power input. Another problem with such a low voltage is that you cannot have a common rail between all plugs, because if one eats slightly too much, all others will fall below 4.5V or even 4V and will be under-spec.
Last and not least, USB is a terrible plug. You always have to try it both ways, you can't visually know if you have to plug it upwards or backwards. And even when you're in the correct direction, you have to approoach very precisely for it to plug correctly. How many of us really look at the plug when trying to insert it ?
I really think that the EIAJ connector has more future. Fit it with 13-20V and make the spec so that it should never go below 13V even when highly loaded. That way, you can design miniature power converters which will be able to provide 12V without much hassle (even 1 single component for the cheap ones). With that as a standard, you could see plugs everywhere including in airplanes, providing unregulated voltage which will fit every usage up to about 65W per plug. And that plug does not need to be looked at in order to insert it. We could even imagine a smaller version for small devices, similar to the common mini-jack found at the other end of USB plugs to bring more power to 2,5" USB/ATA adapters. This would be nice for ipods, mobile phones, etc...
The other advantage is that many equipments nowadays are already compatible with that voltage (eg: notebooks) and will not even require any additional converter.