He didn't pioneer shareware distribution or the free demo model. This is a good write-up on the history of shareware:
http://www.loonygames.com/content/1.24/feat/
(Has this been posted to HN before? Good stuff in there.)
http://www.loonygames.com/content/1.24/feat/
(Has this been posted to HN before? Good stuff in there.)
* "more or less" because the business structure is more complicated than that and there was still an Apogee Software alongside 3D Realms, but for all practical purposes, what was Apogee Games became 3D Realms and then died due to the Duke Nukem Forever project.