I'd hardly call it a killer feature when it's something Teamspeak, etc have had for ages.
The killer feature, i.e that which sets it apart from the rest, is the rich text chat and that it's free.
I thought the killer feature is that it is free and convenient. No one has to do any work to setup a server like with Teamspeak or Ventrilo. It has been a long time since I played video games, but back then those were the two popular options.
