Slack’s not perfect, but here are some features I like:
Emotes let you “respond” to a message in less than a second with zero typing. At CEA, we have an “eyes” emote that means “I’ve seen this message”, which saves me 30 seconds over sending a “thanks for sending this, I’ve read it” email. We have lots of other emotes that stand in for other kinds of quick messages. I send a lot less email at CEA than I did in my most recent corporate job, at a tech firm with pretty standard messaging practices.
Channels act as a proactive sorting system. CEA has an “important” channel for time-sensitive things that everyone should read and a “general” channel for things that everyone should read, but that aren’t time-sensitive. If all the messages on those channels were emails, I’d wind up reading them all as they came in, but in Slack I can ignore most of them until I hit the time in my day when I want to catch up on messages, without spending any energy on sorting.
Slack also has a feature that lets you set “statuses” in the same way the HBR article discusses (e.g. “working on important thing, available after 4:00 pm”), which takes less time than writing an auto-reply and also doesn’t add dozens of automated emails to other people’s inboxes when they try contacting you.