I wish it had color text input support. At my last job, they were looking to replace IRC with RocketChat, and that was a big let down. I even submitted a PR for "Colored Markup" (https://github.com/RocketChat/Rocket.Chat/pull/8639/commits)
The use case at the last job, was that I had an IRC bot that had a lot of stuff to deal with support stuff, like DNS configuration for the Hosted Exchange offerings (2010 and 2013 were offered), and it would do colored text for good and bad configurations. Just made it easier to point out the issues.
Well, IRC clients allow for more customization for the end users, RocketChat - there was very little. The dev team didn't want to add hacks on to it and would only update when RC would release a new version.