Very little changed between .6D and the last version put out for Diplomacy. It's basically just some bug fixes.
Rebellion has added a great deal of flexibility. Terraforming, smarter AI countermeasure usage, independent cloaking methods the AI can use. We were able to significantly improve performance with increased functionality.
Some of the things discussed aren't new however, the X Class planets are in the Diplomacy version, they're much better implemented in the Rebellion version, but a less fleshed out version was already around before the switch.
Most of the work has been in the aesthetic side of things. Some of the work under the hood has been extensive, but the lion share of the labor and improvements have been towards audio and visual additions. Many model improvements, replacements, audio and string work.