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.
