I think what JoshuaFH quite makes sense... or at least it is appealing to me.
So you're basically tricked into unleashing a dark, evil thing upon the world (as well as many new enemies), that cannot be stopped by anything you know of... but then a counterpart to this evil thing, The Hallow (and it's accompanying creatures), suddenly appears. Where did it come from? Did someone or something plant it there, to battle the horrors unleashed by you? Are the creatures in the Hallow hostile to you because you have been designated as an antagonist, on account of having unleashing the corruption?
Something that is also curious is, where are you? You are in a place that is extremely varied; it has deserts, forests, a jungle, massive amounts of caves, access to the Underworld, magical floating islands, and is surrounded by oceans. The oceans suggest that you are on an island, or something similar. What other continents and lands are out there?
And I'm also not sure about the new hardmode bosses. A giant mechanical worm, a mechanical skeleton robot, and two mechanical (or at least part mechanical, they can fire lasers and shoot flames) eyeballs. The worm and skeleton robot, otherwise known as The Destroyer and Skeletron Prime, seem to be quite robotic, as if made by someone. But the twins, they have unique names; Retinazer and Spazmatism, suggesting some form of intelligence, or at least something similar. Where did all these mechanical enemies come from, and the things used to summon them, a mechanical eye, a mechanical worm, and a mechanical skull?
Then there are some of the underground enemies and things. Skeletons, undead miners, an undead archaeologist and wizard, traps and abandoned mine-shafts. All undead were once living, so where did they come from? Who set the traps and made the abandoned mine-shafts? Judging by the fact that it is decently difficult to acquire wire (you have to find the mechanic and buy it from her, you seem incapable of making it yourself), the people who made those things had to be decently advanced. And how did the Mechanic get the wire, anyways, when no one else in the world has it, and when there were some ancient people using wire to make traps and such deep underground?
And the dungeon, who built it, who populated it with all it's enemies, and why is a cursed old man guarding it? Why is the old man even cursed?
The time period seems to be some kind of mash-up between fantasy medieval and steampunk, with some more advanced things thrown in (similar to the game Thief's universe). You have normal swords throughout the game, for the most part, but then there are these Phaseblades thrown in the middle, as well as a space gun, clockwork assault rifle, shotgun, handgun, laser rifle, flame thrower, a cannon that can shoot stars, and machine-guns that seem to be somehow made from sharks. And many of these guns somehow use your inner magical energy.
The best equipment in the game is Hallow equipment, and right below it, the 3 new ores; cobalt, mythril, and adamantine. The equipment you can make with it, the repeater, sword, spear, and such, is not mechanical at all. The repeater is a crossbow, the others are simply spears and swords. The only mechanical thing that comes from the new ores are the chainsaws and drills. Why then, does the best equipment in the game, not have some of the most advanced technology in the game? There are laser rifles and phaseblades, yet the destruction they can cause is beaten by crossbows and swords.
Where did the 3 new ores come from, anyways? Were they placed by whoever placed The Hallow in the world, or who placed The Corruption? Neither the Corruption or the Hallow seems to share any affiliation with you, pretty much everything wants to kill you. So where did these ores come from, these ores that allow you to, eventually, deal with the creatures of the Corruption and the Hallow with ease. Is there a third-party that takes pity on you?
There are so many unanswered questions, so many untied strings. What is really going on in our characters' worlds, and why?