Wave was supposed to kill group email threads, but its interface was too techie and complicated -- and it did not have time to take off.
Unison is the group email-killer we've been waiting for: the speed and realtime discussions of Wave, but with a simple interface. (See for yourself; you can get a free Unison account here.)
Here's how Wave looked, in case you forgot:
We never explicitly sought to replicate or compete with Wave -- but our premise was the same: group email threads are destroying our lives (or at least our productivity at work…). What we're hearing from customers is that their group email usage is shrinking by 70-80%, as they move conversations to Unison instead.
So try it out: get started with Unison.