Terraria is monolithic. There are no games like Terraria, and there will never be another game like it. The game is not perfect, but its execution is so solid and its mechanics so deep that it's no wonder people put so much time into it. RPG and building mechanics are enmeshed in such a way that there is a deeply satisfying feeling of control. The game's pace unfolds at the pace you allow, and so, as your tiny wood hut metamorphoses into a base, and your base into a town, and NPC's come to inhabit your homes, there is always a feeling of verisimilitude, like the part of the world you play in is part of something greater.

The constant comparisons between this game and Minecraft sell both games a little short - but just between you and me, Terraria is better.