Miller was one of the first to show that shareware games for the PC could be profitable. In the late 1980s, shareware had been quite popular for various programs and applications and developers would make whole versions of their software available for free. If consumers liked the product they would pay the developer. Initially, Miller found that this was not a profitable model among gamers, who would generally take what they could get for free without paying. The model that Miller developed that eventually proved very profitable for games was to offer only the first few levels or first episode of a game, and make customers pay if they wanted the full product. This proved especially profitable with the Commander Keen series and Wolfenstein 3D (developed by id software, but published by Apogee), and Apogee's own hit game, Duke Nukem.
Miller is an atheist. He constantly posts updates about his views on religion and humanity on his Facebook profile.