Valve is rolling out a new feature for Steam called "tags". It's a new feature for Steam, but not for the internet; look above this text and you'll see tags for this story, which let us collect and sort posts of a similar nature. Which is exactly what they're going to do for Steam.
The idea is that users can tag games with stuff they think is appropriate. So Skyrim would be tagged "RPG", Call of Duty "shooter", etc. Clicking on those tags would display every game on Steam that was an RPG or shooter. But because users are doing the tagging, there's scope for way more detail.
Mods. Indies. Musical. Nordic. Hand-Drawn. 4x. Grand Strategy. Pirates. Sex. You could use more specific tags to group and discover games that were right up your alley.
I'm trying the feature out now and it's pretty great. Steam has recommended a few key tags already based on my library - strategy, indie, open world - but a further click presents me with more, like "female protagonist", "viking" and "turn-based".
Seeing as Steam's library now consists of thousands of games, and new ones are coming in too quickly to keep track of, this seemingly harmless little feature is actually going to be a powerful tool to help people discover awesome games they may have otherwise missed.
Unless, of course, people find a way to troll it...
Steam Tags [Valve]