The only improvements are thinner, lighter, faster, camera.
The ipad1 is already thin and light enough to carry around. It's not like the difference between a DSLR and pocket sized cameras.
The speed difference may be huge, but ipad1 copes with existing apps well. It's not going to make a huge difference unless app complexity grows multi-fold.
So that leaves the camera, only if you want to do video/Favetime chats in it would be necessary.
Having said that, if you don't have an ipad1 already, it's time to get the ipad2.