I often miss the ability to react to comments (particularly internal notes) in tickets. It seems many of my most-often used tools (e.g. Slack, Github, Clubhouse) have the ability to do so, and I wish Zendesk had it too.
One use case I encountered today. I added an internal note asking a question, and someone got back to me with the answer. I didn't want to add extra noise in the ticket by acknowledging that I'd seen the answer and that it was helpful—I wished I could react with a ✅ or 👍.