I love this. I think the biggest issue with Slack at the moment is on-boarding. It's a little tough to rollout to people who aren't technical. Keeping their attention can be tough.
Frankly, the 'killer app' at my org has been /giphy - people see that, get a laugh and suddenly they're exploring new features.
Also of note: threading of conversations is confusing initially - perhaps a wizard type interface for new users that prompts then when they post ('did you mean to reply to a comment') or even a 'reply' item alongside Reactions would be nice.

Also, after typing this i threw it into my sites /feedback. Cant hurt.