My favorite thing about Slack is how they've thought of all the little things. My go-to example is that when they send you a notification email, it includes links to silence further emails with a single click.

Obviously, this isn't a big "feature." It's not going to show up on their marketing materials. Nobody will ever say "hey, you should try out Slack because they let you silence emails." But it's this sort of UX philosophy, of anticipating and cleverly solving tiny little annoyances, that make it a really pleasurable product to use.