Terraria is much, much more than cheap 2D Minecraft.
Terraria, in it's own way, has a very strong building scene, but it's more about combat and taming your world.
Thank you!
VERVERYVERY ABRIDGED;
Terraria pits you against an absolutely merciless world. It's your job to build shelter, mine for resources, and hunt monsters to get the gear you need to defeat the numerous bosses in the game. They, in turn, drop great loot, and so on and upwards until you're fighting the Frost Moon, which is currently the toughest End Game event.
On top of this, there is a spreading, evil biome called the Corruption (Or depending on the world, the Crimson) that WILL take control of every single block in a world if it is not maintained and controlled.
There's a second half of the game called Hardmode. Killing a boss called the Wall of Flesh, located in the Underworld and summoned by sacrificing your Guide via dropping a Guide Voodoo Doll in the lava, will activate it. It involves new, tougher bosses, the ability to break Demon Altars to spawn new, powerful ore, and a flood of new, powerful mobs. By far the most drastic change is a huge new vein of corruption that appears, and an entirely new, also spreading Biome called the Hallow to contend with. The Hallow in game is portrayed as a 'good' biome, but make no mistake; Pixies, Unicorns, and flying pink bubbles will
not hesitate to gib you.
After progressing far enough in Hardmode, you kill a boss called Plantera. Killing Plantera changes your dungeon, turning up the difficulty to 11, as they say. This new, Hard Dungeon is the source of even more new mobs and gear, some of which is absolutely monstrous in power, but even those items are eclipsed eventually.