Hi, Slack!
Thanks for how you try.
I need you to try harder.
First, I need you to stop stealing my attention and focus. People want to express themselves with animated GIFs and emoji (see #4 below) and stuff. Let them express. But let me control. If I'm reading a post needing my attention and some [wonderful coworker] wants an animated penguin dancing just on screen above or below, focus becomes a challenge (as is my good will toward said [wonderful coworker], but that's another issue).
Second, I need you to be more efficient. Your desktop "apps" are battery vampires because they are not apps. They are your website wrapped up in Google's Chrome browser in something called Electron. While there are some advantages to the arrangement, I value efficiency in message delivery and the battery life to act upon such messages.
Third, searching is weird. There is some syntax around it but is specific to your service. I would not mind so much if it were more or at least as mature as PCRE or Google's search. As it is I find it hit and miss.
Fourth, emoji has its place. In Slack, it's decoration and message medium and "flair" and … More often than not I can't tell what message is being sent. "Huh?" is becoming my #1 response. Let me render them all as text (see #1 above) or block them.
One area where I like what you're doing is identifying bots. In most circumstances bots are useless and I prefer to avoid them. I like that they are labeled so I can ignore them. I would like to be able to block specific bots for me while keeping them available for others.