As someone who has taken an advanced OS class that involves implementing portions of such an educational / teaching OS, I also perceive "educational" to mean "f

As someone who has taken an advanced OS class that involves implementing portions of such an educational / teaching OS, I also perceive "educational" to mean "for learning OS dev". The other side of the spectrum for me is "research" and "proof of concept" OSes though - like House (written in Haskell) or Singularity and Midori.