Many pundits would like you to believe that Terraria is a copy-cat of Minecraft. Others want you to experience Terraria for what it is: a game with similar goals that is partially influenced by Minecraft (and Metroid and Final Fantasy). Which ever side of the fence you fall on, there are some business decisions that distinguish Terraria from Minecraft.
First and foremost, Terraria launched through Steam. Minecraft, while technically not yet launched, opted to roll out of the alpha/beta gates with its own platform and suffered weeks of log in server problems, PayPal issues, and a ton of website downtime. Terraria launched smoothly and pain free through Steam. So while Minecraft developers scrambled to deal with platform issues after their beta launch, Terraria has enjoyed a honeymoon where their developers can focus on the game.
Secondly, I get the distinct feeling that the Terraria developers just want to make a great game. On the other hand, the Minecraft developers certainly have made and continue to improve a great game, but I've always gotten the feeling that they want to be a "big game developer". Notch, Minecraft's chief developer, alluded to this when asked about possibly working at Steam's parent company Valve: "I don't want to work for Valve. I want to be Valve."
All evidence shows that Terraria is a smash hit. Peak user numbers on Steam have been stellar and for a few days even unseated Counter Strike as the most played game on Steam (that's a pretty big deal).
As far as I can tell, it's smooth sailing for the Terraria developers and with the Steam platform (and possibly Steamworks) on their side, I expect many great things from Terraria.
PS. If any Terraria developers read this, please, pretty please implement the Steam cloud for world saves! Thanks.