This is actually a huge + for any extensible chat client. We send alerts from various monitoring solutions and use emoji to quickly identify the environment and app. Makes it very easy to understand exactly what is going on.
Can we call it link unfurling in general? That's what really attracted me at first. (And emoji ofc)
The IRC client I use/d inlining of images and gifs.
“Reactions”, custom emoji, ease of integration (webhooks), and ease of signup are the things I hear quite often.