Get ready for some huge changes (more than just the 64-bit-only one that folks have been talking about!!)
Catalina will have an entirely separate volume for the Mac System, and it will be LOCKED (read only)! So, your system software should be entirely protected from malware, and from random corruption. (This is very similar to how the old Atari ST computer used to store the OS in ROM. To my knowledge, there never was any malware for the Atari ST!
Extensions have been deprecated. Most of your legacy extensions should still work (for now), and you can still install new ones using the right-click-to-launch method, but this will change in time. This is a good thing. You can forget about malicious fake Flash installers and bogus browser extensions that serve up ads in the near future.
Languages such as Python, Ruby and Perl, which used to be included with the Mac OS, now won't be. So if you have scripts or applications that require these languages you will have to download a runtime version. If your scripts or applications won't work with a runtime, then they will have to be re-written.
As part of Catalina's new heightened security, the user is going to have to give a password before an application can access all sorts of stuff, such as your documents in your documents folder. This feature will protect your valuable data nicely, but I suspect that users will whine incessantly about it.
All sorts of hardware peripherals, especially printers, will need all new drivers to work under Catalina. If your printer's developer doesn't already offer new drivers for Catalina for download....DON'T UPGRADE TO CATALINA!!! Many Mac users, especially business people, use Fujitsu's ScanSnap desktop scanner. Older versions of this scanner won't EVER be compatible with Catalina. Newer versions will require a driver update.
I can tell you one thing for certain...users who don't like change are going to be doing a lot of complaining about Catalina. My advice? WAIT. No need to jump on Catalina until you know the repercussions for you personally, and you know that what you need to make a smooth transition is in place.
The Main Benefits of Using New MacOS Catalina for Data Privacy - CPO Magazine