I like the different channels for different topics, the ease of posting photos & documents, the ability to search for past messages, and the general compatibility with lots of other tools. There are myriad bots and integrations that I haven't fully explored yet, but what I've used seem helpful.
There are some built-in features that seem advanced but are surprisingly simple and delightful. For example, you can ask "Slackbot" to remind you about something later simply by sending yourself a direct message, and you can also ask Slackbot questions about how to use Slack. For example, I just asked it "How do I create a poll?" and got a helpful and even conversational response.
I am always learning about new features, too. For example, I just learned that you can make voice and video calls and screen-share through Slack. I had no idea, but now I'll have to try that. It's always a more robust tool than I realize!
Plus, the friendly tone of Slack's whole persona (including their release notes for new updates) and the enormously wide range of emojis (and ability to create custom emojis) make for a fun, expressive experience. The app has all the benefits of standard communication and file sharing software for the workplace as well as the aspects we all love about social media.
Recently the desktop app has been crashing, which is inconvenient. However, you can open Slack on a webpage, so it isn't that inconvenient after all.
Invest time into learning about the various integrations, and work with your colleagues to make sure it's clear what type of messages should be in Slack and on what channels. It may very well work for all your messaging needs, but it won't be much help if everyone continues to use email instead or other apps.
As a company comprised entirely of remote workers, we use Slack for many company-wide announcements and interactions, and it facilitates a very easy, friendly communication. We have some channels that are company-wide so that anyone who wants to subscribe can, and others that are limited to certain groups who will need the information shared there.
A personal favorite of mine is a channel we have set up with an RSS feed that pulls in industry news, competitor content, etc. daily so that we can all learn on the job. This feed sparks ideas and conversation and makes it effortless for all employees to stay up-to-date.