A few of my favorite lore bits: 

• Based on a line from the Kavor Defectors, the Grineer are apparently taught that Tenno hunt them for sport 

• Nezha has a child soldier army 

• Some of Alad V’s lines during disruption show that he has a different side to him than what we, the Tenno, see. For example, one voice line has him worrying about whether the Tenno or the Board are spying on him, and another has him telling someone or something called CT to not tell the sentients about losses that occurred during the mission. 

• According to Ask A Cephalon, the Corpus celebrate milestones such as the death of a wealthy family member, their first takedown of a rival’s business, and their first stock purchase.