I'm not even thinking about the endgame stuff right now. I'm still mucking about with a dirt house after having started a new character in a new (large) world and it's all the little things that are fascinating me.
Like the underground area for the desert that contains sand that my puny copper pickaxe can't touch.
Or the large amount of new desert enemies, some of which are quite dangerous to a newbie character.
Or dicking about underground and suddenly finding railtracks.
The game now keeps track of how many enemies you kill and awards you banners at every "milestone". Want that elusive Nymph banner? Kill 50 nymphs (a.k.a the Mother of all Grinds).
And I may be asking a dumb question here, but...how long ago did Terraria allow one to use the hammer on wooden platforms to alter their direction? I can make staircases now!
I found a cloud island, was lucky enough to have a high hill just underneath a low-hanging island, could see the water rolling off the side. It's so low, not even Harpies spawn there.
Blood Moon events now have new monsters.
Slime raining from the sky is one of the funnier things to happen to me in this game.
I've explored long enough in both directions to reach Crimtane sections that I doubt I'd survive crossing, still haven't found the dungeon or the jungle.