To be honest, there isn't that much new info, other than some retconning, about 60% of it are pretty pictures.

Only thing we did not know about the Faceless revealed in the Compendium is that Malassa sided with Urgash during the Wars of Creation, for some unknown reason. Which meant that the Faceless fought alongside the Demons and this is the reason Angels hate, fear and distrust the Faceless.

About the Southern Desert Orcs of Sahaar it is said that they are the most Nomadic of the 3 Orc people.(The Other 2 being the Steppe Orcs of Ranaar and the Island Ors of the Pao and Savage Islands). They are the least numerous of the 3 and in most ways resemble the Orcs of Ranaar.

Again, most things about Dwarves we already know. There is some cool Trivia about how the Dwarven body temperature is absurdly hot, which makes them resist both heat and cold very well. They do not feel the cold of their snowy realm and can pick up hot coals with their bare hands. Something realy cool is that Dwarven beards combust when they experience strong Emotions. Most memorable about the Dwarves is their allied creatures. Bears we already knew about, but now Valkyries and Fire Giants are part of Fortress too. Valkyries are burning warrior spirits that are born when Arkath amalgamates several Dwarf-souls why died a heroic death on the battlefield. Fire Giants are born a runic-volcano ritual, where Arkath selects a few chosen Dwarves during a volcanic eruption and grants them a new body of basalt and fire. These Giants are rightly revered as the Chosen of Arkath.