Prior efforts were menu oriented or attempts at desktop UI on a tiny screen, like WinCE.
I worked on a Linux based tablet back in '99, that had all of those flaws.
Apples biggest innovation was recognising when the hardware had caught up enough to make this attractive to normal consumers.
That's not a small thing - the entire rest of the industry tried,failed, and wrongly assumed the failure meant the concept was a dead end and mostly abandoned it, before Apple resurrected it.
I think until the iPhone most of the touchscreens were resistive and single-touch. It was multi-touch capacitive screens, alongside Jeff Han's multi-touch demo that captivated everybody's imagination.
(Incidentally, this is the tablet I worked on: https://digitaltmuseum.no/0210211515576/nettbrett - nothing to make you feel old like having tech you've worked on catalogued by a museum; the early versions ran NanoX with a custom Opera port)
I think the big diff was assumption of handwriting as the UI. Latter palm treo had a keyboard like blackberry, I used several generations of those before trying the HTC g1 :).