Think of the best side-scrolling SNES/Genesis game now add a 2d Minecraft on top of it and random spawning enemies!
But at around the 5 hour mark people tend to leave their safe little home and start exploring, and that is when the Minecraft "build" side of the game ends, and the Castlevania, Metroid, Mega Man side of it beings...
The number of enemy mobs in this game is mind boggling, each of them presents a particular challenge the farther and farther you get into the game. Unlike Minecraft this game has guns, and boomerangs, and harpoons, and "phaseblades" (lightsabers) to fight your enimies off with, and even with this impressive arsenal you'll find yourself dying a lot.
Once you're bored of surface level enimies (usually when you craft your first set of armor), move on to underground ones. Then to Jungle ones, then onto the randomly-generated dungeons...
Also, did I mention this game has boss fights? Like a gigantic eye with 10,000 hit points? Or how about random events like a "Blood Moon" when all enimies spawn twice-to-three times as often and can open your doors.
Terraria is NOT Minecraft with more combat, it is an old-school castlevania game with building and randomly generated worlds...