Ok admittedly, this was 10 years ago and 4chan was a much different place (as was the internet at large). I was younger and spent a lot of (read: too much) time on 4chan back then.
At the very beginning of Minecraft, before it was even a thing, Notch would post on /v/ about it almost daily - incorporating changes/suggestions/criticisms people would fling at him anonymously and randomly. No one thought it would be huge - it was just fun. And Notch was so receptive.
I can't help but think his current Alt-Right views may have been influenced by his exposure to 4chan, but in those days it wasn't as crazy as it is now.
I have a 4 year old daughter now who LOVES Minecraft. And personally, it's been valuable to me, and I am utterly convinced that had Minecraft not been developed in its earliest days on 4chan, it might not have gotten to where it is today.
*Small shoutout also to Katawa Shojo - also made with lots of input from 4chan, which turned out to be quite a compleling VN game.
What has 4chan done lately that is valuable? That I couldn't tell you.
For example, couldn't Notch post on Reddit every day, incorporating changes/suggestions/criticisms people would fling at him anonymously and randomly?
Is there something about 4chan/8chan that is intrinsically better for building Minecraft that isn't available anywhere else?
I'm convinced the initial randomness/anonymous feedback/chaos of that initial period helped grow the spark from the ember that split off of Infiniminer.
There is something creative in the chaos of 4chan - just look a greentexts. 99% of them are absolutely terrible. But the occasional gem catches on and made viral on the larger, less vile sites.
Also, I can't believe I forgot about Caturday. That started on 4chan as well. (also 10+ years ago).