This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
Quite a few, although nothing particularly drastic OS wise, mostly improvements although the lifting of the FileCore partition limit and the incorporation of JPEG handling into the OS are quite substantial improvements. Here is the list :-
* Now stored on 2x2Mb ROMs, or an increase ot a potential 4096Kb of OS.
* FileCore improvements allowing at least 4Gb partitions.
* Support for ATAPI style CD-ROM drives.
* JFIF handling incorporated into the OS.
* Standard applications have been moved back into ROM.
* Toolbox modules, the Cv5 support modules, moved into ROM.
* CDFS modules moved into ROM.
* Access modules moved into ROM.