The only very known difference is that Kickstart 1.3 fixes the silly bug that prevented Kickstart 1.2 to boot from a hard disk (a routine in the boot code uses A6 loaded with an incorrect library base, thus ending up being a NOOP and skipping AutoConfig'ed DOSNode's). However, are there other changes?
(This question is only about Kickstart, not about differences on Workbench disks, which were bigger)