You could check out the new feature articles in my blog, links to which can be found in this article. Many others have written about new features in both ACA and AutoCAD; Google should be able to help you find those articles.
Most of the ACA changes since the 2007 release have been incremental. The big interface change will be getting used to the ribbon. Toolbars and the pull-down menus have been dropped from the ACA.cuix, but tool palettes are still used extensively. You will want to read up on annotative content, added to AutoCAD (along with the ability to set a drawing scale for the model tab and for viewports on the layout tabs). ACA content had drawing scale/annotative content prior to AutoCAD (but mostly affecting scale at time of placement); the ACA content has extended that previous functionality by incorporating the AutoCAD annotative feature but keeping the annotation plot size.
The ability to edit Wall Cleanups in place was added, and then refined, during that time. Column Grids were enhanced. Grip edit capabilities for both AEC and AutoCAD objects has been greatly enhanced. The Center/Offset options when placing Doors and Windows were split into separate options and then reunited. Grouping and subtotals were added to Schedule Tables. You can use Layer Keys in Display Settings. 3D AutoCAD objects and the editing therof has been greatly improved.
The above are just a few items that came to mind as I was typing this.