The most useful thing about Slack is you can press Shift+ESC (sometimes you have to press it twice) to mark all conversations as read.
Inbox zero with two keystrokes. Whoever implemented that, I'd like to buy a coffee someday.
(Also, I discovered this because I use Ctrl+Shift+ESC instead of Ctrl+Alt+Del to bring up the Windows Task Manager immediately without going through the intermediary screen that pops up, and one day, I missed Ctrl.)