The killer feature is that nontechnical Mac and Windows users can click a link emailed from anyone on their team who is already using the app, and get seamless group chat across platforms, without calling the IT department to set up a server and/or open firewall ports, or committing someone on the team to managing anything XMPP-related. It just works.

Even Linux folks can participate with additional work (e.g. dozens of packages to install if you're running Ubuntu 64, since Adobe AIR is 32-bit only; annoying).

That said, I agree that the barrier to entry is pretty low.