How are the Spiel des Jahres decisions made?
From the Spiel's website (the English version at least):


  Jury The jury consists solely of journalists who specialize in game
  reviews, have worked for extended periods for newspapers, magazines,
  and broadcast organizations and who have thereby proven their
  competency and ability to make sound judgments. The association's
  charter forbids jury membership to any person involved in any way with
  games publishing and sales. 
  
  Election Procedure The jury will consider all new family and adult
  games published during the current and previous calendar year. There
  is no separate request for submissions. The election procedure goes
  through two phases: an information gathering phase and an initial
  evaluation phase.  After the two phases end, the jury meets in
  conference to create a list of candidates. From this list a shortlist
  of three nominees is selected for the main award. The winner of the
  Spiel des Jahres award is elected the evening before the announcement
  of the award.
  
  Judgment Criteria The nominees for the main award and the special
  awards are judged on the following aspects:
  
  
  Game idea (originality, playability, play value) 
  Rules design
  (structure, clarity, understandability)
  Layout and presentation (box,
  game board, rules) 
  Design (features, workmanship)
  


So the jury are pretty powerful as far as I can make out.  But hopefully unbiased!