Lots more items/resources, lots more monsters, bosses.
Items include accessoires (e.g. Cloud in a Bottle - let's you double jump; Rocket Boots - let's you fly at the cost of mana...oh yeah, there's mana), weapons (swords, magic, guns), the usual set of tools, and more.
Different areas in the game, like dungeons (strong monsters, rarer items), floating islands (not much content there, yet, will change), and more.
Yes, it's more action based and fast paced.
Still, it's an early release. Many changes and new features to come (hopefully).
Edit: If you don't mind spoiling the experience go to the Terraria Wiki. Incomplete and not very precise so far, but it'll work.