Designer front ends matter. You have no screenshots, which is going to severely limit adoption. If the design is average or subpar, it will limit adoption.
Also, integrations with third party services matter. Everyone seems to ignore this, but it's critical.
Uhm, there is a screenshot in the README, both for mobile and a desktop browser. Maybe we could move it up a bit more. As for the design, sure, you're right that it matters. We try to keep it as clean and intuitive as possible, but none of the core devs is a designer, so we just do it as best as we can. Ideas for improvement (or even pull requests) are always greatly appreciated!
For third party services, Glowing Bear has a number of plugins that embed various content right in the conversation (images, youtube, spotify, etc). Not sure if that's what you're referring to.
The uhm is unnecessary; I don't see any screenshots on the GitHub page, linked or otherwise. The page linked from the website is https://github.com/mattermost/platform.
For third-party services, I'm talking about things that matter for getting work done. For example, issue trackers like JIRA.
Also, integrations with third party services matter. Everyone seems to ignore this, but it's critical.