While not technically a feature, one of the things in Galactic Civilizations II: Dark Avatar that long time players will notice is the significant improvement in graphics quality.

Military Starbase in GalCiv II: Dark Avatar

VS.

Military starbase in GalCiv II: Dread Lords (original)

These kinds of improvements are present across the entire expansion pack.  Moreover, because of improvements in how we did the textures, they actually use less memory.