According to the new policy, it seems that Valve will be limiting the access and features of certain accounts unless they have spent at least $5. Presumably this is to ensure that the user is an actual gamer who might have bought a game or two during their time on Steam, or at least made some in-game purchases even with free titles, like DotA 2 for example.
That being said for those who have their accounts limited, some features that they will not be able to access to includes opening group chats, sending friend invites, voting on Greenlight, Steam Reviews, and Workshop items, participating in the Steam Market, gaining Steam Profile levels, using the browser and mobile chat, and more.
Now there are some users who might be using Steam for more social purposes instead of just playing games and are no doubt a little annoyed by this, but if you have yet to make any purchase, you can always opt to put at least $5 into your Steam Wallet to remove the limited account status.