I think a good amount on info on the changes and improvements can be found in the release notes for the first code versions of 6.0.x.x and 6.1.x.x. I have never seen a matrix for this. Of the top of my head some features that I believe were added at some point in AOS 6 were a management dashboard, device type classification, dns named acl's and other.
The compatiblity matrix for AP's can make a case that some newer AP's are not supported on 5.x.x.x. See link below:
http://www.arubanetworks.com/pdf/products/MAT_AP.pdf