There are various pre-made forum systems on the internet, like XenForo, phpBB, myBB, vBulletin, etc., but I’ve always liked Discourse more because of the various extra features that the other forums generally don’t provide (and I want to thank the Pretendo admins for choosing Discourse as well).
For example…
- You can unstick stickied threads for yourself, which is actually really useful for QoL as then they won’t clog up space at the top even though you’ve already read them.
- The poll builder has cool features, like automatically closing your poll, choosing whether the votes are public or not, or even restricting them to certain roles.
- Notifications are reliable and quite customizable, which is a win over certain legacy forums where you barely get notified about anything.
- Timers! Ranging from basic slowmode to things like temporary thread locking, threads that lock themselves after a certain time, etc. I’ve never seen these anywhere else.
I also used to host a Discourse forum one time. Unfortunately it was extremely short-lived as my $100 in Azure credit only lasted me a few months, but it was amazing to see how customizable the Discourse software actually is. And yes, you can also make a forum private so you need a registration key to create an account 
Is there anything specific that you guys like about the software?