Steam may be adding a report system to its store, according to a recent update to the Steam beta. Under this system, titles may be reported for breaching law, being offensive or containing hate speech or pornographic material.  The full list can be seen below.

It's not currently clear what the punishment will be if a game gets enough reports. Certain reports, such as being offensive or containing adult content, could be applied to a reasonable number of games on Steam, and thus are unlikely to lead to a game actually being taken down. Reports, such as fraud, harmful material and legal violation on the other hand, would presumably have harsher  consequences.

This is likely part of Valve's move towards a more open, public managed storefront, which they have been discussing for a while.