It was a little bit of everything, really. Discord exploded in popularity because:
1. Its free functionality was more generous than many comparable services. Nobody wants to pay for a Mumble server.
2. Its UI and audio quality and noise cancellation settings put much of the competition to shame.
3. Only needing one account for every Discord server in existance gave it the same kind of appeal that let Reddit/Facebook kill off most individual forums.
4. Good marketing, which gave it the critical mass of users and hobby groups that it needed to succeed initially and now make it harder to move away from.
Speaking from being very early on the train for Discord, it also had an extremely solid userbase right from the start because much of the early pre-marketing pull into it was for raid groups in the then-new Final Fantasy XIV: A Realm Reborn. People really needed a chat client to coordinate a big number of players and it was totally free, functional, and new on the scene. It spread a ton in the community and the people working on it were players as well from what I remember, though it's been a long time. So as the game grew in popularity, and everyone who was in a large group was starting to use Discord more, it cemented friend groups that formed in the increasingly popular game along with the Heavensward release and helped solidify a foundation in the gaming community imo.
Discord's killer feature for my money has always been that ever user in a call gets a slider bar (and mute button) to control the volume they hear ever other individual user in the call
Zoom, in 2025, still makes you wait for the host to figure out which of the 30 other people has a dog incessantly barking or is causing the echo or has horrible feedback, and then try to talk them through fixing it before finally muting them.
We had that in Teamspeak/mumble/vent, but the chat functionality on those platforms were definitely an afterthought. Not a place you'd cultivate a community.
Teamspeak had a ton of friction. When I was playing FFXIV and made my discord account, someone literally just sent a link in the text chat of our Free Company, I opened it in browser, made an account and used it right there immediately. To timestamp this, it was the summer of 2015, a little under a decade ago, iirc.
Not sure about now, but back then Teamspeak meant installing an application, setting it all up, having someone in the Free Company (almost always more than the limit for a free server on TS at the time) pay for a server or self-host (even more friction). With Discord there was no debate or decisions, just one step: Click link, sign up.
Its killer feature was how frictionless it was to adopt initially. If I sent you a Discord link, back then you could just click it and be going in under a minute.
Same reason Zoom quickly took over video chat. It was so easy to use that you didn't have to convince your friends to sign up for it, you just sent them a link and it just worked.
Yeah, didn't even verify emails. I got a surprise discord account back in 2015 because someone else used my email. I guess this xkcd[1] applies to younger people too.
Those tools were all, comparatively, trash. Literally ALL of the gamer groups I was familiar with switched en masse. You can’t compare Mumble and Discord IMO. There was so much setup friction that anything was better than what we had at the time.
Gaming was becoming less and less the domain of the tech-savvy crowd, strongly curbing the public's appetite for such host-it-yourself services. Teamspeak/Mumble were already dying at the hands of (inferior) free/easy chat platforms like Steam & Skype, so it's really no wonder that Discord was able to swoop in and clobber all of them by simply being free and featureful.
The UI is awful, Discord volume isn't exposed, no option to do it, either, so you have to adjust game audio or individual user audio. They put the whole kitchen sink in, for some reason, too, there's a text chat in the voice rooms that needs to be user-revealed for some reason, instead of being exposed by default. Most of it is unintuitive at first use.
Then there's the many contraindications e.g. privacy policy, walled garden, and the dog shit internal indexing and the fact that it isn't externally indexed, there's a lot of pertinent information on there from skilled individuals that could serve society a la the BBS era that will never be surfaced again because it's now being posted to Discord, though that's tangential. I hate it, but it's easy enough to use, though negligibly dofferent compared to Skype, which had many of the same issues.
I think the biggest attractor for my friends was being able to idle in a server so it was easy to start a party vs starting a call on Skype which requires a little more arrangement. Lower cost less friction.
Weird, I have never noticed a single one of the issues you mention. There really isn't much I don't like about discord, though I wished threads worked more like Slack.
You cannot search in discord threads. Until recently, there wasn't even a way to go to the first message in a discord thread without scrolling up manually.
I like that they are unobtrusive and low cost. Easy to spin off a small discussion without bothering others.
In discord, a thread becomes a big visible thing in the sidebar. More than a couple and you're annoying everyone on the whole server. Completely the opposite of what I want.
If I haven't used discord in a while I will get stuck in a voice chat and cannot for the life of me figure out how to exit. I clicked on the channel name to enter, but I can't exit from there. Clicking on other channels doesn't get me out etc etc.
Just funny to find myself knowing I've got this wrong before and being annoyed at myself _and_ at them
If you can handle a ton of users on not much hardware it’s quite easy to offer a generous free tier. The more “high touch” (resources per interaction) your SaaS is the more VC money you’re setting on fire trying to achieve a network effect.
And during a downturn the high touch services lose customers faster because part of the virtue signaling of cost cutting is choosing cheaper options that take a bit more work.
1. Its free functionality was more generous than many comparable services. Nobody wants to pay for a Mumble server.
2. Its UI and audio quality and noise cancellation settings put much of the competition to shame.
3. Only needing one account for every Discord server in existance gave it the same kind of appeal that let Reddit/Facebook kill off most individual forums.
4. Good marketing, which gave it the critical mass of users and hobby groups that it needed to succeed initially and now make it harder to move away from.