Well the "big" HD support thing was in the 2.05ROM which AFAIR predates 3.0 and would only effect the IDE part of "scsi.device" as A3000s came with 52 or 105MB drives stock.
A3000s came with HD floppies so I'm not sure it really got added that late. And even if the code in 2.1 would have been just a SetPatch bringen 2.04 ROMs to 2.05 functionality both still "2.0".

CrossDos was just a 3rd party add on, so that would leave only locale as major backported feature.

Still wonder why they didn't add 3.0 2nd party piece (datatypes)......