Where is the user created content on Steam? I think you're missing a vital element of the gaming infrastructure. Before I begin, let me say that I use Steam daily. I use it to play games, watch trailers, and download demos and retail software. Recently you've launched the Community section and I appreciate the potential of this service, but in it's current form it is lacking a feature that is a big hitter.
We are awash in Web 2.0 buzzwords, hype, and copycats, but the core concept of Web 2.0 is community participation. Users have control to generate, shape, and rate content on Web 2.0 sites. A good example of this is paradigm for Web 2.0, YouTube. Videos on YouTube can be manipulated in a variety of ways by users. They can rate them for quality, add them to personal playlists, post videos in response to other videos, and comment directly to a video. Those are only a handful of features that make YouTube the best example of community built content. So where does Steam fit into the mix?
The defining function of Steam is to provide content on demand. So it is perfectly positioned to support user created content. What would make this feature better than web sites already hosting levels, mods, and other add-ons is the ease of use and community driven organization. When a gamer downloads from a web site, the file must be unzipped then moved to the proper game directory. Through Steam, once the download is initiated the next user action is simply launching the mod. The framework is there, the community just needs the tools and on-ramp to begin creating the environment.
The benefit for you of course is more eyes on your product and for longer periods of time. As we've seen with sites like YouTube and Digg, users who take an active role in their community develope strong bonds with it. What price can you put on customer loyalty?
UPDATE 7/17/2013: Finally getting around to updating this because for about a year now Valve has basically implemented everything I've suggested via the Workshop. I'm just going to take full credit. You're welcome Internet.