First off the new apps (music, podcasts, etc) are terrible. They killed off iTunes but replaced it with much worse. These apps don't behave like standard macOS apps, the UI is full of inconsistencies and is just so empty. This website has nice examples of the failures of modern Mac OS: https://annoying.technology
For some reason after updating the "new updates" badge was stuck on the system preferences icon (and even on the preference pane itself) despite no updates being available. I ended up having to delete a plist and reboot to fix it, apparently a common issue.
The Mail app will now randomly play the "new mail" sound. I can't confirm it for sure but I'm assuming it's treating read, existing mails when they are moved to the trash/archive or newly created drafts. They screwed up the mail app, a problem that has been solved for decades. WTF? The worst is that I see no major changes in there, so why touch the mail client in the first place if you're not even going to give me additional features in exchange?
Xcode was stuck upgrading in the App Store. It would start the process and never make any progress. Cancelling it had no effect. Rebooting cancelled it but the second attempt, while making progress, ended up failing with a generic error message with no actual information. Logs are useless because they're being spammed by all the background processes even during normal operation making it impossible to find anything. Finally the third attempt succeeded.
1Password now takes 5 more seconds to unlock my password database. Somehow this disgrace of an OS slowed down the password hashing process by an order of magnitude.
Switching screen resolutions or connecting to an external screen takes a good 10 seconds of flickering and frozen UI before everything starts working again. This is now actually worse than both Windows and Linux. I dread moving the laptop or touching the USB-C cable (also because USB-C is so brittle) when it's connected to an external monitor out of fear that it'll disconnect/reconnect and I end up in a 30-second cycle of flickering.
I upgraded a couple of days ago, so those are not early bugs. Apple had a year to fix all of this. The Xcode thing might be an isolated issue but there's no excuse for the general performance penalty or the stuck update badge which has many hits on search engines suggesting it's a widespread issue.