Pencil improvements: I'm more of a doodler and a note-taker than I used to be, but I still don't use the Apple Pencil that often. But for those of you who do, there are plenty of Pencil-friendly updates to be found in iPadOS. The palette of tools that pops up when you start using the Pencil has been redesigned, and you can flick it around the screen so it doesn't obscure whatever you were doing in the first place. Apple has also said that, thanks to some under-the-hood optimizations, the Pencil's latency has dropped to as low as 9ms. With that said, I haven't noticed a huge difference in Pencil performance so far.
For what it's worth, one of the smallest changes to the Pencil experience has been the most useful. If you swipe in from the side of the screen with the Pencil, the iPad will take a screenshot you can immediately start marking up. You can even use this feature to take full-length screenshots of web pages and documents.
Sidecar: One of the most interesting additions to iPadOS is the ability to connect iPad Pros to Macs for use as secondary displays or graphics tablets. My colleague Dana has the Catalina update running on an iMac and took the feature for a spin herself. You can read her impressions here.
Mouse support: At long last, you can use a mouse (wired or Bluetooth) with your iPad. It takes a little setting up, though: You have to enable it as a pointing device in the Assistive Touch, which requires a trip into iPadOS' accessibility settings. Scroll wheels work fine right out of the gate, and if your mouse has additional buttons you can easily customize their actions. Keep in mind that iPadOS and iOS aren't really optimized for interacting with mice, so the feature isn't quite as helpful as you might hope. Since there aren't any sensitivity controls, the cursor just jets all over the screen even though I'm barely moving my hand.
It'll be a few more months before Apple releases a final version of iPadOS and based on what I've seen so far, it's going to be worth the wait. The company's business customers should be especially excited: Apple addressed many of the criticisms that prevented the iPad Pro from being the do-it-all computer it aspires to be. As for everyone else, they'll benefit from subtle performance improvements and some extra polish. There's something for everyone here, and since Apple is one of the last major players in the tablet space, that's good news indeed.