iPad apps will run unmodified as basically virtual floating iPads which is nice, but not anything too special. What's much more interesting is that existing iPadOS apps can be transformed into first-class visionOS apps by just checking the "visionOS" checkbox in Xcode, which gets the app the full 2.5D treatment with all of the various controls taking on the expected visionOS appearance and behavior.
This means that the Vision Pro is probably going to have by far the best selection of day-1 apps that any AR/MR headset has ever had. Most of these apps won't be anything revolutionary of course since they're adapted versions of existing apps, but it makes the platform much more practically usable.