Reactions in chat
Slack has reactions (not emojis in messages) in private chats. We use them quite frequently for acknowledgement, approval, understanding, etc.
Mythu Siva commented
This is so simple yet critical to having a great experience.
It's takes mundane chat room experiences to a more real-time and engaging conversation. It also addresses the problem with not exactly knowing how everyone is feeling - they would need to verbosely type something rather than just reacting to a message.