I see a lot of customers that implement a version of AX and make modifications and then have to maintain them. One thing to keep in mind that at each version AX is always adding new functionality and now with the policy changed on CU’s including new features there is another chance to get new capabilities and perhaps remove some of the customizations you did.
This document is a handy one as it lists the new and change features in the AX2012 versions.
It’s a long document but the sections are well structured by version and application area. If you are on for example AX2012 RTM and looking at going up to AX2012R3 then read each section for example on general ledge from each version and CU to see the accumulation of features added.