Seems to me that solving the problem of the safe mass-production, storage and transport of gaseous and liquid hydrogen would be the game-changer, for terrestrial needs as well as space travel. It can be gotten from water or even air. If the danger of handling could be reduced, it would surpass petrol as the primary fuel and then off we go. Solves the carbon problem and the energy resource problem.
Then if someone figured out how to better harness the sun's energy... Of the energy that reaches earth, what do we capture and put to use now, like .001%? Or hydroelectricity from the tides? Lot more reliable than winds. All the current problems/roadblocks seems to be energy-related.