I think the reason discord got so popular is because of the ease of use. Being able to join a server with a link (and not even having to download a client if you don't want), being able to see who's in voice without "connecting" to the server like mumble or teamspeak, persistent chat. This is what is unique about discord; all those other things came later. This is the user experience the alternatives need to replicate. Custom emojis are not a deal breaker, the hangout experience is.