What is currently driving me crazy about slack is that:

1) Slack constantly makes subtle changes the UX that always takes a bit of time to figure out, I don't want to read update notes to find out how a critical comms tool has changed eg. yesterday the mac chat became psuedo wysiwhg and it broke my muscle memory and stuff bounces around while I'm typing, eg2. Drafts: FU, I didn't ask for you to reorder my UI around so I can't find stuff, DON"T CHANGE MY MENUS

2) Bugs, I left hipchat because they couldn't stop breaking stuff with every update. Many of Slack's changes are "breaking stuff" until you figure out what they've changed.