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.
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.
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.
With a wider range of programs to choose from you can be more specific and choose the best, most economic program for your needs.
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.
With a bigger drum you can fit in larger items such as duvets, or do a big load at once.