Slack made it easy and fun to connect with your coworkers, it wasn't unique, but it was a right time sort of thing.

We had chat systems before like IRC, Hipchat, Campfire, Flowdock (We have used them all), but with Slack it was the first time I felt a push from other departments to use it.

I'm still sad about the implementation of threads that Slack went with, Flowdock did this much, much better, every new post into a thread were also posted in the channel view but with a unique colour, the interaction can be seen here: https://youtu.be/BxRE5GUbFes

I also liked that Flowdock had the concept of an "inbox", so every channel had their own inbox for integrations that wouldn't post a message and notify everyone, but you could still chat with your team about a certain item in the inbox.