i think what could really make the difference is the video chat feature though. if properly implemented...does it work fluently?

Yes. We're veterans of previous video startups. This is video meeting done right. It's not a big part of the product but it is important coz it means you don't need skype or hangout or whatever in addition to this.

I'm looking to write an open source app with basically similar video chat functionality. Could I ask what's the best way of implementing this?

I'm guessing WebRTC would be the way to go... once it is released at least