I've always been a big fan of 2D platformers where you use the mouse to aim and shoot. As a kid, I was heavily influenced by games like Worms, Liero, and Soldat. I've always wanted to make a big RPG game in that style, but how does one guy go about designing a world large enough? Well, everyone's favorite feature of SMBX(another game I made) was the level editor, so I realized that leaving it up to the player to make their own world was the best way to go about it. Then Minecraft came along and showed me how to put it all together.
Updates are a ton of work, but they are very satisfying. I am always a nervous wreck anytime I have to click the button to make Terraria go live, but it's all worth it to see so many people play the game and to read their comments about it.