The very difficulty in measuring productivity inevitably (humans trying to conserve their effort) leads to immense variability in productivity.
And also leads to difficulty of talented new people getting started; I read a very interesting experimental economics paper using oDesk on this yesterday: http://www.onlinelabor.blogspot.com/2012/02/economics-of-cold-start-problem-in.html
And also leads to difficulty of talented new people getting started; I read a very interesting experimental economics paper using oDesk on this yesterday: http://www.onlinelabor.blogspot.com/2012/02/economics-of-cold-start-problem-in.html