Machines with a load sensor can weigh the load and adjust settings accordingly. This means you don't have to feel as guilty about doing a smaller load as the machine will use less water.
A faster spin speed means you can get things cleaner with a lower temperature of the water. Using a fast spin also extracts water, meaning that the clothes come out nearly dry. Generally, anything over 1,000 rpm is very good.
More weight capacity helps if you want to wash something heavy like curtains. A traditional household machine can hold around 6kg, so with a machine much bigger you may waste energy using it when only half full.
With a delay start option you can schedule the machine to begin at a later time. You can also set the machine to wash at night if your electricity is cheaper outside of peak times.
A really quick cycle can be a great option if you are in a hurry.
With a lower noise level for washing you can worry less about disruption and may be able to do a wash at night or while reading.