His innovation was to create the modern online verification bullshit that plagues the industry. Before Steam, you bought a game, put in the disc, and played it. If it HAD and online component, you may have had to type in a CD key.
No extra launcher to access your library. No having to reconnect to the internet to renew your license. I had a dedicated gaming PC that didn't even have a modem. And no mandatory updates. It's unacceptable that any time I want to play a single-player game I have to hope there hasn't been some 140 gig patch that I have to install because the multi-player suite I'll never open added a new $30 outfit to a character and they just make everyone reinstall everything.
If Half Life 2 had come out a year earlier, I wouldn't have been able to play it because I didn't have internet connected to my gaming PC until college.