A new post on the Steam Blog discusses "Who Gets To Be On The Steam Store" in light of recent questions about Valve's policies regarding games that include adult content and controversial topics. It's a lengthy discussion of the issues involved and the history of their policies. They then announce that their new policy going forward is to allow all content "except for things that we decide are illegal, or straight up trolling." Here's their conclusion:

So what does this mean? It means that the Steam Store is going to contain something that you hate, and don't think should exist. Unless you don't have any opinions, that's guaranteed to happen. But you're also going to see something on the Store that you believe should be there, and some other people will hate it and want it not to exist.

It also means that the games we allow onto the Store will not be a reflection of Valve’s values, beyond a simple belief that you all have the right to create & consume the content you choose. The two points above apply to all of us at Valve as well. If you see something on Steam that you think should not exist, it's almost certain that someone at Valve is right there with you.

To be explicit about that - if we allow your game onto the Store, it does not mean we approve or agree with anything you're trying to say with it. If you're a developer of offensive games, this isn't us siding with you against all the people you're offending. There will be people throughout the Steam community who hate your games, and hope you fail to find an audience, and there will be people here at Valve who feel exactly the same way. However, offending someone shouldn't take away your game's voice. We believe you should be able to express yourself like everyone else, and to find others who want to play your game. But that's it.

In the short term, we won't be making significant changes to what's arriving on Steam until we've finished some of the tools we've described in this post. As we've hopefully managed to convey, navigating these issues is messy and complicated. Countries and societies change their laws and cultural norms over time. We'll be working on this for the foreseeable future, both in terms of what products we're allowing, what guidelines we communicate, and the tools we're providing to developers and players.