The Death of File Systems by Jakob Nielsen, February 1996
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When I go into my Gmail account, I have filters set to label my email. I don't use folders there. Half of the documents I care about are on Google Docs, where I use search instead of a file browser. I don't use a filesystem metaphor on my non-jail-broken iPod Touch either, except maybe to access the apps.
With mobile web apps set to explode in 2011 -- Android is predicted to go to sub-$100 retail price, it means a lot of people's first experience of a personal computing device will not be a filesystem.