In an attempt to combat the creation of new accounts for phishing and spamming on Steam, Valve are changing the requirements for fully 'activating' a new account. Now in order to get access to things like the sending of friend requests and voting on Greenlight the account in question will need to have have purchases of about $5.
Before then all accounts are created as 'limited' accounts barring them from sending friend requests, chatting with other players, voting on Greenlight and Steam reviews and more. The $5 barrier can be passed in a few ways though, the most simple being the straight purchase of a game worth that much or more. Other ways include gifting a game of that value or more to another player or by adding $5 to their Steam Wallet.
In a recent post on the ruleset change, Valve wrote: "We've chosen to limit access to these features as a means of protecting our customers from those who abuse Steam for purposes such as spamming and phishing. Malicious users often operate in the community on accounts which have not spent any money, reducing the individual risk of performing the actions they do. One of the best pieces of information we can compare between regular users and malicious users are their spending habits as typically the accounts being used have no investment in their longevity. Due to this being a common scenario we have decided to restrict certain community features until an account has met or exceeded $5.00 USD in Steam."