What is the difference between slack and a normal, old school forum?
Both have threads, PMs, Images, Emoticons, etc.

Is it just the UI that got improved?

Notifications, integrations (with lots of apps), webhooks to your external tools (like CI tool output)

To me, it reminds me of IRC in a modern way

Improved?

Yikes.

Margaret Hamilton did lunar landing with kilobytes of ram, slack requires 6 gb just to deliver useless garbage like “omg sarah you dating brayden now omg” and “@ all 30-minute sprint ends in 5 minutes, 2 hour retro will be at the room 6” while still be lagging terribly no matter how much ram I have

You don't have to hit F5 to get the new messages. That's probably the most critical difference.

It’s more inclusive by being bi-directional

The server/account separation feels more intuitive for work related comms than eg. discord or irc.

The biggest advantage over irc is how I don't need a BNC to be able to receive messages while offline, but I absolutely loathe the styling and how you can't customize it without basically dropping in entire css hacks. I prefer minimal UIs to keep noise at a minimum so you can imagine what my irc client looked like, and how I felt moving to discord.

The old one probably works faster on desktop as it isn't made of Electron js crap.