Coming from the 2016 model, the most compelling upgrades for me are FaceID, the new pencil and the smaller overall size of the device. FaceID is just fantastic, both for unlocking the iPad and for using apps like 1Password. It’s really a significant part of the device for me (the more I use it, the more annoyed I am that my Mac doesn’t have it).
The new pencil is great, and the fact that it’s always charged and magnetically secured to the top of the iPad means that it’s far more of an everyday tool than the old one was. I’m not an artist, but I still get a ton of mileage out of the pencil when I’m editing text, taking notes in GoodNotes, etc.
The smaller size makes the new 12.9” a lot more portable than the old one. I find myself taking it with me more often (though some of that may also be attributable to the Sutter Tech Sling).
There’s a lot of more minor improvements (the new Smart Keyboard, industrial design, better display, etc.) but I think FaceID, the pencil, and the smaller size are the big ones. To me, they made the upgrade more than worth it, but YMMV.