Darkness Rising introduced Champion weapons (much like epic armor), player mounts (horses), Champion Levels and subclassing (small abilities from another class), new dungeons and instanced zones for the new Champion Quests and new graphics for the game world's models (such as barns, haystacks and forts, this includes the Darkness Falls dungeon and the capital cities) which are following the design ideas presented in Mythic's new graphics for the starting cities. Darkness Rising was also the first paid expansion to Dark Age of Camelot able to be downloaded. A trailer has been created by GOA to showcase this expansion. Mythic also introduced a new island, called Agramon, which acts as a central island that connected the frontiers of the three realms. Unlike the rest of the Frontiers zones, there are no keeps and any player from any realm can open the gates that border the island.
Source: Wikipedia, "Dark Age of Camelot", available under the CC-BY-SA License.