1.number of cycle programs
With a wider range of programs to choose from you can be more specific and choose the best, most economic program for your needs.
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.
With a bigger drum you can fit in larger items such as duvets, or do a big load at once.
During a spin, machines can get quite noisy. With less noise the machine is not as distracting and you won't be restricted to using it at certain times.
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.