The making of Elite(theguardian.com)

  But while they were still sorting out the mechanics of combat and docking, they 
  started to worry about the adequacy of what they'd be giving the player (whom they
  imagined, of course, as a hypercritical consumer like themselves, bored with Space
  Invaders after the first brief rapture). It wasn't too hard to come up with a 
  solution. They could keep the player interested by letting them upgrade the weapons
  on their ship to ones that made bigger bangs and allowed you to use different
  tactics. But this little alteration perturbed the universe of the game. The classic 
  action game of the early 1980s - Defender, Pac Man - was set in a perpetual present
  tense, a sort of arcade Eden in which there were always enemies to zap or gobble,
  but nothing ever changed apart from the score. By letting the player tool up with
  better guns, Bell and Braben were introducing a whole new dimension, the dimension
  of time. They were saying they wanted the player to hope, to scheme, to plan. Also,
  to play for much longer than a slam-bang 10 minutes. And that was only the 
  beginning. The solution threw up a further problem, as each of their solutions
  would. How would the player get a bigger gun? They should earn it, Bell and Braben
  decided. No free lunches in this universe. But that implied money, in a set-up that
  a moment before had existed quite happily as an economy of pure explosions. "We put
  a bountyon the pirates. Then we thought even that would become quite samey..." They
  kept following the implications of each invention until they arrived at another
  invention. A money economy with more sources of income than just bounty for shooting
  pirates implied trading. Suddenly, the player's spaceship wasn't just a nimble 3D
  firing platform: it was a cargo hauler as well. And trading implied places in which
  to trade. The game needed serious three-dimensional geography. And things to trade.
  And prices. And markets... The new wishes multiplied. They kept going.