I know it's off-topic but I think one thing that helped Steam was the 360 using DVDs instead of HD-DVD or Blu-ray. This helped keep file sizes of games reasonably low(less than 9GB) as a standard whereas now games come in at the 40-45GB mark.
Had games been such a large size it might have hurt some appetite for digital distribution. Keep in mind that download speeds back around 360's launch were nowhere near as good as they are now.







