Two quotes, one common theme:
It's like sitting a kid down at the ORIC-1. Kids are naturally curious. They love blank slates: a sandbox, a bag of LEGOs. Once you show them a little of what the machine can do they'll clamor for more. They'll want to know how to make that circle a little smaller or how to make that song go a little faster. They'll imagine a game in their head and then relentlessly fight to build it.
-- Technology - The Atlantic
malleability of environment is also a key characteristic. Even the most simplistic of farming games on Facebook ranks higher on the “affect your world” scale than World of Warcraft does… and this sort of personalization of the environment is standard not only in social games but across the social web today.
-- Raph Koster - Are virtual worlds over?