One second-order effect is that countries with good infrastructure planning would reap a significant relative gain. The fast train from Paris to Nice would become faster yet, but would trains on the Acela corridor?
Next in line: Desalinating water would become cheap and easy, enabling the transformation and terraforming of many landscapes. Nevada would boom, though a vigorous environmental debate might ensue: Just how many deserts should we keep around? Over time, Mali and the Middle East would become much greener.
How about heating and cooling? It might be possible to manipulate temperatures outdoors, so Denmark in January and Dubai in August would no longer be so unbearable. It wouldn’t be too hard to melt snow or generate a cooling breeze.
Wages would also rise significantly. Not only would more goods and services be available, but the demand for labor would also skyrocket. If flying to Tokyo is easier, demand for pilots will be higher. Eventually, more flying would be automated. Robots would become far more plentiful, which would set off yet more second- and third-order effects.
Cheap energy would also make supercomputing more available, crypto more convenient, and nanotechnology more likely.
And limiting climate change would not be as simple as it might at first seem. Yes, nuclear fusion could replace all of those coal plants. But the secondary consequences do not stop there. As water desalination became more feasible, for example, irrigation would become less expensive. Many areas would be far more verdant, and people might raise more cows and eat more beef. Those cows, in turn, might release far more methane into the air, worsening one significant set of climate-related problems.
But all is not lost! Because energy would be so cheap, protective technologies — to remove methane (and carbon) from the air, for instance — are also likely to be more feasible and affordable.
In general, in a carbon-free energy world, the stakes would be higher for a large subset of decisions. If we can clean up the air, great. If not, the overall increase in radical change would create a whole host of new problems, one of which would be more methane emissions. The “race” between the destructive and restorative powers of technology would become all the more consequential. The value of high quality institutions would be much greater, which might be a worry in many parts of the world.
This is a thought exercise, and I would say you are wasting your breath if you fume against fusion power in the comments.