Steam is different in that:
1. Your games are tied to your account, not your device. You can run Steam from any computer that meets the system requirements and play your games anywhere. Whenever I go visit my parents for instance, I log into Steam on their computer to play some old school games. If I want to buy something, I buy it there and when I leave and go home, it'll be waiting for me to download on my home computer.
2. There is a trade off for not being able to resell games; you can buy them much cheaper. During the recent Christmas sale for instance, you could have bought THQ's ENTIRE catalog of games for $30. I picked up Sleeping Dogs and XCOM - both games released in 2012, for a combined price of $16. Right now on Steam, you can buy the entire Alan Wake franchise for $10. What's the trade off on consoles? Seems to me like you pay the same for less features.
3. There is competition to Steam. Green Man Gaming, Origin (ha ha, just playing), GOG, Gamers Gate, Desura. There is no competition to Xbox Live or PSN.
4. There is no universal DRM. Some games don't have any (any game sold on GOG), some games have an always online requirement (Diablo 3) and a lot of games have limited forms of it like Steamworks. Most games generally speaking allow for offline play.