As you know, we use socket.io to power our real-time aspect. They've been on 0.9.16 for the better part of a year (perhaps more), while the team (a small one, like ours) has been working to refactor the underlying engine to a more stable, secure, and powerful one.
Off the bat, it generally means one thing -- that those users who can't access NodeBB for whatever reason due to their ISP (websockets blocked, etc) will no longer experience a "delay" before NodeBB starts working*. Some users (less than 1%) can't even use a NodeBB at all, because the mechanism that drops a websocket connection back down to XHR-based doesn't kick in.
Both of those problematic cases will be resolved when NodeBB is upgraded to use socket.IO v1.0.
* The way this works is: NodeBB loads, and attempts to establish a websocket connection. If after a delay, a connection isn't made, it falls back to xhr polling. Socket.io 1.0 will do this in reverse, xhr first (works in 100% of cases), and upgrade to websockets.