Lumbar support, height/back/armrest adjustment, headrest, right fit for your body. In other words, features that promote good posture, instead of slouching. Nowadays I find most ergonomic chairs have pretty much the same features, and what differentiates one from the other the most is quality. Aeron and Steelcase are basically BIFL chairs, whereas a cheap chair from Staples or Amazon may last you a year or two before things start falling off.