Where Fourth Wing set up the world and characters of the series, Iron Flame significantly expanded these. Like, doubled+. Wish we'd had a character list to reference, including

gryphons and fliers

because your girl was lost. I still don't know/remember who half the dragons and their riders are! I've rated Iron Flame lower than Fourth Wing because there were small plot moments I didn't like, but overall, I think (hope) that it'll be a good bridge to the rest of the series. There are so many places it can go now!