Steam profile privacy settings have received an overhaul, giving users more control over what aspects of their profiles are shown to the public. In the announcement, Valve says that these newly implemented changes and upcoming ones are results of feedback it has gathered from the community.
In addition to all the previously available options such as profile and inventory visibility, the settings page is now home to the 'Game details' category. This option gives Steam users the ability to hide what games they own, what they have wishlisted, their achievements, as well as playtimes. Also, they can choose to stop broadcasting what game they are currently playing.
Beyond that, an independent option to hide total playtime for games regardless of other privacy settings has also been made available. In Valve's own words, this is so that "You no longer need to nervously laugh it off as a bug when your friends notice the 4,000+ hours you've put into Ricochet."
Note that following this update, all Steam profiles' game details option is set to 'friends-only' by default, and this has not bode well for third-party websites that make use of this information to track various stats. One of the biggest casualties of this change seems to be Steam Spy, a popular website that uses profile data to estimate game sales numbers, which will no longer be functional.
As for more profile changes, Valve is working on implementing an invisible mode that will be available alongside to the current options of online, away, and offline. Once a user sets their presence to invisible, they will still be able to access the friends list and use Steam chat, but they will appear as offline to outside users. The invisible mode will hit Steam's beta client soon.