I require something with good back support. The back needs to be really comfortable - if the chair doesn't make me want to lean back, then I unintentionally lean forward which is bad on my back.
I use armrests, because it's important that my arms are elevated above the desk surface half an inch or so, so I'm not accidentally resting my wrists on the edge of the desk. My hands go down half an inch to rest on top of the mouse and keyboard.
Also important is elevating my monitor high enough, or lowering my chair low enough, that I am not constantly bending my neck to look down at my monitor. I want the center of the screen to be about eye level. A couple textbooks under the monitor stand works fine for me.