The most engaging part is the world of 5 factions. The whole large "evil" scheme to overthrow the government is not that impressive, but the underlying political values, ideas of peace and war, morals and ethics, bravery and idiocy, truthfulness and thoughtlessness, are what keep me with the book.
Little romance going on is nice to add. Some touching moments I appreciate, especially about family. And I have to say I am in love with the "freedom" that original Dauntless faction values, so I might be a little bit biased here.