Earlier this week Valve introduced Steam Guard Mobile Authenticator as a means to keep its users safe from phishing attempts, and now it's taken another step in that direction. From here on out, until you spend a minimum $5 with your account certain features are blocked. What're you going to miss out on? Friend invites, opening group chat, the Steam discussion boards and voting on Greenlight games among other things. But, considering that most people use the service for, you know, buying and playing games, this really should only affect those who're actively using the service for nefarious purposes.
Valve says that the easiest way to identify malicious users from folks who are genuinely using the service is comparing spending habits to account age. "Typically, the accounts being used for [phishing] have no investment in their longevity," Valve writes. So, if you're a typical Steam user you shouldn't notice a difference aside from a hopeful decrease in spam in your inbox.