IMO vs Discord

Although IMO has a comparable, and in some cases superior, feature set when compared to the competition its key selling point is its interface.

From LifeHacker when it won "Best IM Client":

Signing on, navigating chats by tapping the tabs at the bottom, searching for buddies, and virtually any other operation is easy to find and use. The app, itself, is very quick. Even sending a voice IM, if you don't feel like typing, is responsive and sends quickly. Everything imo does it does well, but it's excellent navigation really makes it top notch.

IMO also has a fully functional website that you can use for chat while on your desktop or laptop. This really puts IMO ahead of the competition as you can use the same system on all your devices so you don't have to deal with overlapping notifications etc. (Note: IM+ also has this feature)

It should also be noted that it has the highest rating in the App Store due to both the UI and its stability.

Tabbed chats make it easy to switch between conversations

Both your buddy lists and chat histories are searchable.

Concurrent sessions allow you to be logged in on your desktop while also being logged in on your phone.

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.