IRC, freenode, #hurd, 2011-07-26:

< antrik> Tekk_`: regarding microkernels: the basic idea, and really the
  *only* fundamental difference, is that they isolate things in separate
  address spaces. everything else goes back to this.
< antrik> benefits from the isolation generally fall into two groups: more
  robustness (main focus of Minix3), and more flexibility (main focus of
  Hurd)
< antrik> while it might also encourage some other good design choices,
  these are secondary effects: such choices can also be implemented in a
  monolithic architecture -- and not necessarily harder. just less obvious
  in some cases...