Just enough power. Diesels run most efficiently at higher loads (like all internal combustion engines), and the 6 hp Lister is just right for an energy-efficient household.
Fuel efficiency. In my actual experience, a 2000 to 2500 watt load (average for a fuel-efficient household, including charging batteries for generator downtime) results in about 5 hours runtime per gallon.
No oil pump. Less to go wrong. If an oil pump is not needed, why add complexity? The way this engine is designed, splash oiling is sufficient for the bottom end. The only thing an oil pump would add is pressure lubrication of the top end (valve train); and because of the low maximum speed of 650 RPM, valve spring pressure is low so the valvetrain doesn't need continual lubrication. Just like an old ship engine, there are lube points that you hit with the oil bottle as part of the preparation to start the engine for the day or, in the case of continuous running, every few hours.
No water pump. Less to go wrong, less parasitic drag. This adds efficiency. It's called theromosiphon. Just set up a water tank or radiator at the proper height relative to the engine, and let the natural laws of thermodynamics go to work.
No thermostat. None needed. Some people who own Listers add one, but do you know what has been the most common failure I have experienced in engine-powered equipment? That's right, the thermostat. They generally (in my experience) fail in the closed position, resulting in engine overheating. Theoretically, adding a thermostat gives higher efficiency and perhaps longer engine life due to slightly higher running temperature. In reality, a well-designed cooling system and load management does the same thing, without the added failure point. These engines have a long-standing reputation for lasting 100,000 hours between rebuilds, and they gained that reputation even though they didn't have a thermostat from the factory.
Dirt simple. Anybody that has any business running a homestead can understand how this thing works, and how to keep it running.
Simple injection pump. No $1000 pump rebuilds; this is a common Bosch pump that can be rebuilt for a few bucks.
Well-suited for alternative fuels. The slow combustion cycle and cast-iron piston are tolerant of alternative fuels, including filtered waste vegetable oil that has not been processed into biodiesel.
Heavy flywheels for surge capacity. The 6 hp output is capable of about 3500 watts continuous output with an ST generator head, but if you oversize the generator head (mine is 7.5 kw) it will start inductive loads like a standard generator of twice the capacity, because of the energy stored in those huge flywheels. This is really handy for running air conditioners.
Low speed. 650 RPM. Besides the long life and ability to efficiently run on alternative fuels, the slow speed means the sound it makes is actually pleasant, and is easier to silence if you don't want to hear it.