I can speak for the ACL conferences (disclaimer: which I volunteered to organize a couple times)

Streaming the talks, having questions via chat, putting all papers and posters online, multiple chat rooms, this is all there. Content-wise you can access everything. And since remote participants don't eat, their fee is a bit lower. There are also experiments on making poster sessions more interactive, but no one wants to use shared headphones in a loud room to talk to the (remote) authors.

The real challenge is: how do you get those attending in person to interact with those that are virtual only? Will you take out your phone and hang around in the chat rooms during the break?