What annoys me about stuff like Slack is that it's misused. It's made for small teams but I've been 10k people open source projects use it instead of IRC. Of course, it was laggy and they eventually couldn't afford it.
That was the biggest annoyance I experienced at companies that based their communications around IM clients like AIM or Gtalk. My previous company switched to HipChat at some point and everyone showing up as their real name instead of some stupid username was a surprisingly huge improvement.