Maybe I just don't get it.
You have a pretty nice and extensive character creation kit, and can choose between small, medium and large worlds. There's plenty of customization from the start.
After you get set up, Terraria plops you on a two-dimensional world with a few tools and graphics reminiscent of old NES RPGs, especially the characters. Standing around is an NPC aptly named "Guide" who gives you a basic tutorial, which is unfortunately a very bad one. The things he explains really don't even scratch the surface of what there is to do in the game. Some of them are out-of-place for someone just starting the game, too, as he talks about some more difficult higher-level things when you don't even know how to make a sword.
Assuming you can figure out how to make a shelter before nightfall comes, you'll get your first taste of the combat except for the slimes that hop around at all times. You'll be absolutely mobbed with zombies and flying eye demons, but they're easy to kill. Should you be overwhelmed, the consequences of death are pretty slim. When you die, you just respawn at the first place you came to life (or at your bed once you make one) and lose a little bit of money.
You can explore, mine for ore and treasure, get better gear and fight tougher monsters, and build little towns that merchants will move into. The game is certainly not wont for things to do if you can bear to spend the trial-and-error time to figure out exactly what you're doing.
All of this combined still led to a boring game to me. As I've already stated, I don't think the game is bad. It's well done except for the crushing learning curve for what it is. It's like puzzle games or point-and-click adventures in the sense that you have to be a certain sort of person to enjoy them. I'm just not one of those people.
It isn't the lack of direction or the difficulty of learning to play. It's just the aspects of the game. You can only have merchants in your village -- I'd much rather have villagers who will help you fight and have little quests to give you, more like a violent Animal Crossing. I'd like more varying monsters, more things to do than mine and build (I play Minecraft and I'm pretty sick of the repetitive digging in dimly-lit mines) or even a more involved crafting process. Actually crafting items instead of collecting the materials and being near the appropriate assembler would be really fun for me. I also wish the combat was less simple and had a little more customization to it.
If you like games that are well-done listless cakewalks (and I mean that without sarcasm) then you're going to like Terraria. Just be forewarned, it's not a game everyone can enjoy.