Michael Kerres talks about the factors that contribute to Moodle’s success as a learning platform. The sound file is available as an MP3 download near the bottom of the page, called dll-pod2.mp3 (in German).
He lists 3 main hurdles that Moodle has cleared in terms of acceptance by different users of Moodle: system administators, teachers, and students. For each type of actor, the main attraction is ease of use: easy to install for sys admins, easy to set up a course quickly for teachers, and easy and intuitive to use for students. Make it easy and they will come. 😉