voice is one of the killer features - simple, instant voice chat, just give someone a link and they're pretty much in. no mucking about with settings. in gamer circles that's huge, and the IRC functionality made it great for game guilds or group of friends for async messaging.

compare discord to say teamspeak or ventrilo before it, where you had to both set up the software and configure it, as well as rent or own a server for people to connect to, discord was a godsend

and miles better than skype