As an (ex) developer, it becomes a blur over a repeating 18-month development cycle, but I know a lot of bugs were fixed and platforms like Text Explorer added (a lot of others too, but I was involved with that one...and I'm not sure which release it first appeared in.) Many platforms get new features, and many JSL functions get added. And other things; I know there has been a push to improve error messages, for example.
The list @jthi points at is the best resource.
It is possible you'll find a script that ran in 9 that no longer runs in 14, but those are fairly rare and tech support (or this forum) can help you with those. That mostly happens when a breaking change has to be made to make JMP better: more consistent, easier to understand.
I'd encourage upgrading because the tech support folks are going to be most familiar with more recent versions.
Craige