We ship new improvements monthly, so if you're on an older release you could quickly add a lot of features: emoji reactions, GIF selector, custom emojis, Slack-compatible keyboard shortcuts, hashtags, slash comments, Slack-compatible integrations, etc.
I really don't like the current way of "copy old directory somewehere, copy a new one and replace some files from the old one". That is very painfull and this process does not really invoke a lot of confidence.
edit: typo
I should be clear, my own UI complaints are minor, but I want to acknowledge that people expecting a 1:1 copy of Slack won't exactly find it. Mattermost works well for us, but I recognize that an enhanced UI with tight media integration must be important to many people, otherwise Slack would not have become as popular as it has.
Threading is really the only gripe I have personally - I don't like how it works, and I don't use it much because of that. But then I also don't have any real observations about how it could be improved either, and not using it (and basically operating as if it's IRC) is a perfectly fine solution!
For instance, I try to search for "pass" hoping to dig out the password someone pasted and it shows results containing words like "wordpress" and "css" that petty much buries the actual matches even if they're somewhere way down in the result.
That said, even though it works as advertised the product is not perfect. The Mattermost UI is bare bones and some things like conversation threading are implemented poorly.