The headline feature is one-click matchmaking, both for new games and existing ones such as Counter-Strike. You'll be able to jump straight into a game with players of your skill level, with no history of griefing, by pressing a single button. It also lets you form a party with your friends, and automatically find a game that you can all jump into and play together. There are parallels to Microsoft's Live service, obviously, but the differences are heavily in Steam's favour. You need the £40-a-year Gold subscription to get Live's matchmaking, whereas Steam's is free, and it'll be supported by masses of our favourite games from day one.
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Members can see when others are playing a game, join them, or schedule a match for a specific date and time. They'll also have a dedicated chat channel to talk to each other on regardless of what game they're playing, even between rounds.

Lastly, but perhaps most visibly, Steam Community gives you a personal gamer page that's accessible via the web, with embarrassingly detailed stats about what you play, how much you play it, and what kind of player you are. It even highlights noteworthy trends - the example we saw told the user "You like playing as a Boomer. Sometimes you (accidentally?) shoot your fellow Survivors."