What does fusion give us that existing nuclear power plant tech doesn't?
The energy generated per unit mass in a fusion reaction is ~9 times that generated in a fission reaction[0]:
Considering the mass of the four protons/hydrogen
nuclei (4.029106u) and the mass of the Helium
produced (4.002603u) we get a mass difference of
0.026503u or 24.69MeV. So it is easy to see that
fusion reactions give out more energy per
reaction. However, the energy per unit mass is
more relevant. This is 0.7MeV for fission and
6.2MeV for fusion so it is obvious that fusion is
the more effective nuclear reaction.
The phrase "electricity too cheap to meter" is likely somewhat hyperbolic, but in comparison to pretty much any other mechanism fusion is enormously more productive and efficient.
If we found a way to extract 10x as much energy from coal as we currently do, electricity from coal wouldn't become 10x cheaper, nor would we build power plants 10x as big.
Sure. Specific energy (or energy per unit mass) between different types of materials makes a huge difference. For example (Source here[0]):
Material Type of generation Specific energy (MJ/Kg)
Hydrogen Fusion 639,780,320
Coal Oxidation 24.0-35.0
You're right. Cost and availability play into this as well. There are estimated to be ~1.06 trillion tons of coal on earth[1], hydrogen is the most abundant element in the universe and even makes up a significant amount of the mass of coal.
Burning hydrogen/hydrocarbons is, compared to fusing hydrogen, an incredibly inefficient process.
I'd say that being able to generate 200,000 times the energy per unit mass is an important consideration.
As for availability, hydrogen is more abundant and cheaper to produce (unless you have petatons of plant matter, the right conditions and a few tens of millions of years at no cost to you) than any fossil fuels. Or just about anything else.
[0] https://en.wikipedia.org/wiki/Energy_density#List_of_materia...
[1] https://www.worldcoal.org/coal-facts/what-is-coal-where-is-i...
Edit: Clarified availability.
[1] https://en.wikipedia.org/wiki/Fusion_power#Radioactive_waste
Water is more abundant than Uranium?