I wouldn't say it added nothing. Maybe not as much as Gen 2 or 3, but there was still the physical/special split and the new 3D style graphics. They were the first games to have online and use the touch screen too, which were pretty big.
Gen 5 added a few cool features, like seasons, but the gameplay didn't change very much from HG/SS. Actually, I still prefer HG/SS's gameplay just because the interface is so much better.