My group of friends migrated from Messenger to Discord a few days ago, we are very pleased. Here's a list of what I love the most:
* When a friend arrived in the evening and the others had been talking about random things all day long, the friend was often submerged by the amount of fluff to read. Discord solves this in two ways: 1) the different channels allow us to keep discussions tidy, we have a channel for random discussion, one for our evening plans, one for future holidays, one for music... 2) we can pin (not sure of the name in English) important messages, so the friends arriving later only have to read those.
* The desktop application feels light-weight and fast, even on my weak chromebook. Compared to the memory hog that was Messenger in the browser, it's very pleasant.
* Many fun functionalities such as custom emojis. The emoji panel that opens when you type a colon makes it easier to find the reaction you want to send. I'm aware many on HN don't see the point of emojis, but I find them really fun and useful.
* Bots. Good luck having a bot in a Messenger discussion, with Discord it's trivial.
* The audio discussions are very easy to use and efficient. I love the push to talk mode.
* Muting is fine-grained. You can mute one channel, an entire server,...
* The reactions system makes it trivial to very spontaneously start a little poll. "Where do you guys want to eat? [Letter A emoji] restaurant x [Letter B emoji] restaurant y", and then you react to your own message with both emojis. Your friends just have to click one of the reactions to add theirs, so the little numbers next to each reaction act like a poll counter.
* You can edit your own messages, with no time limit AFAIK. Very convenient, especially with the up arrow which automatically edits your last message.
* Roles are precise and easy to use. We have a role for the core group of friends, with our private channels.
A few drawbacks :
* No quote responses, like Whatsapp has for example
* No attachments larger than 8 MB, which makes it impossible to send even a small video you've shot with your recent phone (unless you decrease the film resolution in your phone's settings I guess)
* I've had a few issues with the Android client, such as receiving a notification for a message, opening the app and the message isn't there. No way to force a refresh, so I have to kill the app and open it again. Might have been caused by battery saving mode, not sure.
* Accessing private messages is not very intuitive. Why do you have to go to the main screen first? I don't have many servers, so I'd like to be able to add my closest friends in the server list on the left of the screen, to access our discussions in one click or press.
* Some clever things are part of the client instead of the server, which would be fine if it were consistent between clients, which it's not. I'm thinking of the +:reaction_here: shortcut to quickly add a reaction to the last message in the discussion. It doesn't work with the Android client.