Fun with friends.
It's easy to say Terraria is 2D minecraft, and hell I think I've said that to several people who have asked me about it, but it's much more than that. It takes minecraft concepts and puts a real game around it (a fun one at that). You do all the minecraft things like chopping down trees, smashing up stones, and building a shelter, but you end up exploring massive dungeons with boss fights, discovering underground forests, and using your jetpack to fly a floating island. This game has depth!
In line with traditional adventure games your character advanced by getting better items. You can craft armor for different slots (Head, Chest, Legs) each changing the way your character looks. Some pieces of armor when combined even grant you set bonuses such as increased run speed. Your character can also hold 4 accessories. These accessories have a wide variety of bonuses like the ability to double jump, swim, or fly.
Unfortunately combat is one of Terraria's weak points. Sword swinging feels clumsy, and this leads to you getting hit more often that you really should. They get around this by giving you weapons that reach further, and by having a host of ranged weapons which feel much stronger. The multiplayer is laggy at times, though I have a feeling this i because most "servers" are simply running side-by-side with a game client.
The desire for finding that next chest or crafting that next piece of armor will keep you digging for hours longer than you planned. After just the first night of exploring and building with friends I feel I've already gotten my $9.99 out of the game, and can’t wait to go back for more. I'm not sure if playing through the entire game by yourself is as compelling, but you can certainly accomplish the same tasks. Overall I would highly encourage you to try Terraria.