Steam is a lot of things - a hub of community content, news, mods, hilarious gifs - but mainly it's a shop. Still, signing up never cost anything - but Steam is no longer 'free' in the way it once was. Valve has announced measures to curb spamming and phishing by introducing a minimum spend on the store.
As of right now, Steam users have to have spent a minimum of £3.35 ($5) in the store, or added an amount equivalent to that into their Steam Wallet, to be able to access all of the platform's features.
It's hoped this simple move will curb spamming and phishing accounts, as no spam/phisher wants to spend $5 on every single one of their dodgy accounts. Unless they do, in which case they are probably a stupid spam/phisher. Entering CD keys, receiving gifts and activating retail keys in Steam don't count towards the £3.55 - you have to actually, actively spend that money in the Steam infrastructure to pass the simple test.
Features that are unavailable until you've plonked down a few quid are as follows:
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
Considering the amount I've spent over the years, I should have my own wing dedicated to me in Steam. Just sayin'.