The height should be adjustable in order to provide proper lumbar support. Both horizontal and vertical adjustments are recommended. Properly designed armrests help support upper arms, back and neck.

TL;DR What makes a chair a chair is its ability to be sat on. An object is only in its category of objects so long as it fulfils its defining use to people. A chair must afford sitting to be a chair, or it seizes to be a chair and is reduced to nothing but void material.