Some of Slack's most under-rated features that are my favorite are the custom emojis that you can upload and the Huddle feature where you can easily jump on a quick voice call. Makes working remotely and async much easier, not to mention building a community.
I use it every day, and it keeps getting better and better, although I do wish that some of their more powerful features were still available for individual users that aren't on a professional plan like me.
