My understanding of the 1.3 content:
-Glacier biome found underground and above ground with new enemies
-New big drill, possible endgame tool
-Retextured nano + cog armor (could mean many new resprites with updated textures)
-New skeleton hellish boss (possibly related to new undead event)
-New goblin invasion enemies including a king boss/miniboss
-New biome of sorts with textured green grass with jungle-like trees
-alchemy class with new flasks & plague doctor-esque armor
-Purple slime crown item of some kind
-Paradox Cohort event
-New music boxes for new music tracks
-New stone boss possible related to paradox cohort
-New weapons including a powerful bow and maybe a shadowflame spell
-new pygmy-esque enemies unlike any other creature in Terraria
-Night of the undead event (similar to pirate invasion and goblin invasion)
-Hunger system + food
-Tremode