USB only gives 2.5 watts, though some people put 5 watts (1 amp) through it in cell battery chargers.

The goal is a data protocol on top of the power, so that the endpoints negotiate voltage and current, thus no risk of damaging parts.

Not quite as good, but still better, would be something like the tips on universal power supplies, where the tip tells the power supply what voltage to use. One could imagine notebook vendors having a power plug which is actually the master 4 or 5 wire connector of one of the tip systems, and telling people "Just get a standard universal power supply. We don't provide one." I would not be too upset because it meant that I could now share powers supplies with others, use stations that provided the universal power, and use the supply I do buy with my next computer, too.