Doesn't make as much sense on Slashdot. Steam is a service that offers a catalogue of games, including some free ones.
Slashdot basically offers one product that's free/ad-supported, and the ability to post is directly tied to that service (not necessarily so the ability to chat/voice-chat in the games).
However, I could see somebody coming who offers a third-party service web-presence validation service. $5 for a validated account, which then could be used to login to others such as Slashdot etc via OpenID or something similar. I would actually be willing to pay for that if it could get past the initial growth hurdles.