True, but you had to pay for a server for TS/Vent/Mumble as well, so the $5/month Discord charges for the higher voice quality is comparable (or cheaper).

Discord is voice chat, with text as a nice extra, and with image/file sharing and screen sharing and emoji/stickers/profiles (these are attractive to some people). IRC is text only. TS/Mumble/Vent were all voice chat, with really awful text chat interfaces (TS was sort of OK, Vent may as well not have had text chat).

I don't think Discord is good as an information archive, and I'm not terribly fond of seeing FOSS projects using it as a primary means of communication, but it's great for gaming compared to what we had before.