Valve has implemented a new policy for freshly created Steam accounts that limit their functionality until they've either spent $5 on the Steam store or have $5 in their Steam Wallet. The full list of restrictions are below:

Sending friend invites
Opening group chat
Voting on Greenlight, Steam Reviews and Workshop items
Participating in the Steam Market
Posting frequently in the Steam Discussions
Gaining Steam Profile Levels (Locked to level 0) and Trading Cards
Submitting content on the Steam Workshop
Posting in an item's Steam Workshop Discussions
Accessing the Steam Web API
Using browser and mobile chat

At first glance it looks a little harsh, but anyone who's met a Steam scammer or spammer should be breathing a sigh of relief. Fantastic!