Steam has become THE Standard in digital distribution, and that is because of good management. The service itself is free, has a good community integration with profiles, chat and voice-chat, groups, recommendations, and more. Also provides development framework for steam integration to developers resulting in games using the steam community, achievements, cloud storing of settings and save-games, server browsing.
The only drawback is: Steam is DRM. A game bound to your account will be bound to it, permanently. You can play games offline and deny updating, but you’ll probably stay online and up-to-date with its nice auto-updating.
Valves (creator of steam) distribution policy is not too coplex and not unfair allowing for publishers and indie companies to publish their games in fair terms. For the users there have been and will be an infinite amount of weekend deals and Summer and Winter-deals where you can buy countless games for a hell of a price.