Mine's definitely the music, I'll always be a huge sucker for that.
For me it is the Bestiary and Void Bag. There's a ton more I love about the update, but those two alone give the game a huge quality increase to me.
I've always struggled with inventory management and storage so having the void bag to pick things up when my inventory is full is fantastic for letting me explore longer.
I also love the Bestiary a ton for letting me see all the details for enemy stats and getting an official description about them.
My favorite part in 1.4 was when a fairy took me to a treasure chest that triggered like 20 different traps on open.
Summoners finally get a worthy "ultimate" minion staff. Yay! (But good lord it's hard to get. Was malding the entire day trying to kill the Empress. Thank heavens for YouTube.)
The new NPC system. With the introduction of the NPC happiness system. It really makes you not want to torture your NPC's and actually makes you care about them a little bit more. On the plus side you also get discounts.
We love everything here,i love the new pets,have a nice day...ove the new mounts, as well as the new npcs. Especially the zoologist, she is a great and unique addition. I like the bestiary progression unlock system