Modern furniture design features sleek, straight lines with smooth and shiny surfaces. The focus is on simple geometric shapes rather than heavy ornamentation. The objective is to create an uncluttered look, free from chaotic lines and color schemes.
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.