The appliance is either Energy Star Certified, or has an EU energy efficiency rating of A++ or A+++. This means it consumes around 20-30% less energy than is required by the minimum local standards for energy efficiency.
Beko WMB 71232 PTE
Hoover OPH 716DF
Machines which use less water are good for the environment and can save you money in utility bills.
Hotpoint WMFG 8337G
Whirlpool AWE 8840
Front loading machines are good as you can use the space above and sometimes stack a dryer on top. In general front loading machines use less energy, using less water by only partly filling the drum and obtaining higher spin speeds which clean without requiring as much heat.
With a transparent door you can look inside the machine and see what laundry is inside.
A faster spin speed means you can get things cleaner with a lower temperature of water. Using a fast spin also extracts water, meaning that the clothes come out nearly dry. Generally, anything over 1000 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.
A really quick cycle can be a great option if you are in a hurry.