Comment by vegai

There has already been a vastly simpler system that predates OS X: Linux or BSD combined with X11 and an arbitrary window manager. Just stay away from anything that calls itself a "desktop environment" and you should be fine.

Personal recommendation: http://www.archlinux.org & http://awesome.naquadah.org/ -- a nice compromise between easy configurability and the "suckless" simplicity mentality.