IRC vs Discord

The IRC protocol is public and open, it is mature and stable, and has been implemented in numerous projects. This means that it can be used freely, for both open-source and commercial projects.

IRC offers choice. Choice in clients (mobile, desktop, web), in scripts those clients run, in servers and features those servers have. You can even set up your own server.

Compared to other common solutions, IRC can be ignored when you are occupied doing something else. Most clients don't pester you with sounds or distracting visuals.

Every notable IRC client supports mentions - notifying you when your name is mentioned in the chat. The mentions in IRC are comparably better than Discord. Mentions used in Discord are easily abused; this doesn't happen in IRC. If someone's talking about you, you get a notification in IRC. This doesn't happen in something like Discord.

IRC has been around since 1988, so there is a ton of existing software to work with it. Everything from clients, to chat bots, to bouncers is already built.

You can choose your favorite server for an IRC network.

No need to sign up for access, confirm your e-mail address. Just choose a nickname and you're done.

IRC is user friendly if you use something like hexchat or kiwi webchat.

Direct Client-to-Client allows users to share files in a similar way to torrenting.

Twitter hashtags came from IRC channel names.

Discord follows the same type of interface design popularised by Slack, which is extremely clean and attractive, and doesn't clutter the interface with unnecessary chrome and cruft.

Unlimited amount of users.

Users can invite others to join a Discord channel via a web link, which means no installation is required to use the software - which is one of the big typical barriers to entry for social software.

Discord allows inviting people to a specific channel with a link. Links can be set to expire. Low effort/passive invite mechanics.

You can quickly send invites to people and they can join in using the web client without having to create an account or install the app.

Behaves and looks like Slack for regular text channels, but with the ability to create voice channels as well

Deny access, Deny write message, Allow read messages etc.

Discord provides an API for creating custom bots.

Discord has a wide variety of bots for things from airhorn noises to automated server management, to leveling systems for chat. The bot API is open and there are dedicated sites for finding bots.

Discord shows which game each user is currently playing, which among other things helps users to form PUGs with minimal hassle.

For those that are used to the way Slack (a similar chat client) works, Discord will be familiar in its use, which lowers the learning curve quite a bit for those that have switched or tried Slack before.

The support is limited but still exists. It's a killer feature for dev teams.

Discord prevents users from being DDoSed out of games.

Targeting roles allows for quickly requesting someone from a group e.g. artist, moderator, etc.

Ability to create channel categories for channels to sit inside of.

Free account have no tangible restrictions.
Premium account is just a way to say "thank you" to developers. And it work for all servers of discord (not as in slack). And it can be purchased by one who wants. In contrast to Slask, where all active members of the server should be "premium".

Have localization for many language, including Russian.