I would say the things that make it eco are energy consumption in use, how eco friendly production is and how good the quality is (how long it lasts).
If you can manage to only fill the amount of water you need then I think that is one of the most important things when you use it.
A quick check on Which Best Buys and Ethical Consumer shows that Morphy Richards Intelliboil 43870 comes up nearly on top on both rankings. I has three temperature settings (100, 95 and 85 degrees) so you can set it to heat up to your specified temp which obviously saves you energy if you choose the lower ones. Also great for green / white tea which should have around 85 degrees rather than boiling:-)
Hope that helps:-)