I'll try to summarize
1. The show kinda glossed over what happened to former slave city Yunaki by saying the Wise masters would keep it slave free as long as the fighting pits were re-opened. In the book there's a whole sub-plot where the former slaves of Yunaki named one of their own King, but he can't maintain control of the city. Dany has to send Daario and some Mercenary companies to support him and they still end of losing which is a major symbolic loss for Dany
2. The whole Sons of the Harpy sub-plot was a lot more complicated. Instead of feeding the heads of the major houses to dragons, she takes their children as hostages. There are 2 or maybe 3 people acting as advisor to Dany in Mereen besides the Hidzo guy. And they all have their own agendas. She had other suitors besides him too. In facy Xaro Xhaxos Daxos (sp?) makes an appearance in Mereen (he's not locked in a vault in the books) and ends up making trouble for her later on.
3. Basically She's got enemies at every turn and everything is converging on Mereen. All these mercenary companies aare trying to get in on the action. There's one subplot I won't go into detail about in case it still happens on the show, but lets just say other people from Westeros to meet up with Dany in Mereen before Tyrion gets there. There's one other group trying to get there too when the book ends.
Bottom line, shit is all fucked up in Mereen both inside and out before she gets on that dragon.