The editor is basicly the same.
There a a couple of new things you can select from say a waypoint window but that's about it (there are a couple new waypoint types and formations)..
The scripting has evolved with tons of new commands to play around with. Check the
COMREF for commands you don't recognise
Also the sqs style of scripting is considered 'dead' in ArmA as it now prefers sqf scripting (you can write 'normal' scripts with the same syntax as sqf functions in OFP)..
But sqs still works just fine..
Are the OFP resources still valid to ArmA
It depends on the resource.
For example if a script uses some object from OFP it will not work in ArmA unless that object is changed into an object from ArmA.
And obviously any OFP resource example missions will not work..