The aspect of fighting bosses and having a clearer progression is pretty cool, but being 2D anything you build is much less interesting. In my main Minecraft world (though I jump around a lot) I have a keep with a courtyard, and under one of the towers there's a rail system that heads out a glass tunnel through a neighboring large body of water to an outpost I have a ways away. You just can't make those same kinds of things in Terraria. Minecarts would work, and I think would be a nice addition (though there's much less need for that sort of thing in Terraria), but you can't build a courtyard, or a true tunnel through the ocean, or similarly your own version of Rapture (as in Bioshock's Rapture).
That said, Terraria is quite fun, and I think has a lot of future potential. Because of the way you save a character separate from the world I can see them adding many tiers of content and difficulty beyond what's currently there. Like perhaps giving the option of making a "Lava Lands" style world, and in order to survive even standing around there you would have to bring an Obsidian Skull, and their weakest mobs could be more dangerous than skeletons from the dungeons. And then maybe at the highest tier of that area you'd be able to make something that would enable reaching a new area that's the next step up, and so on.