Gladwell popularized the "10,000 hours rule", but he didn't invent it. It's originally from a paper by K. Anders Ericsson on "deliberate practice" [1]. It cites a ton of other research in the area and is definitely worth reading if you liked Outliers.

[1] http://projects.ict.usc.edu/itw/gel/EricssonDeliberatePracti...