If you didn't entirely understand how upcoming not-sequel Terraria: Otherworld [official site] differed from Terraria then congratulations, you're as dumb as I am. Fortunately, some GDC footage and a more detailed breakdown of stuff and things from the developers has made matters a little clearer.
The main takeaway is that Otherworld's a little less random than Terraria, although it's not at all a fixed narrative. Your play style will affect your skill with certain weapons, and in turn 'early' weapons can become more powerful, rather than have your future success depend on whether you can lay hands on something beefier.
You'll also be trying to 'purify' the world rather than simply wander around it. Fortunately it means purity in an battling horrible slimy evil sort of way, and not in a telling everyone who's ever engaged in pre-marital sex to repent sort of way. Though there's always room for metaphor, I suppose. However, the 'corruption' (metaphor?) will fight back, which means you need to erect Purifying Towers (yeah, definitely metaphor then) guarded by Wartowers to defend liberated land. And this means tower defence.
Always a risky gambit - tower defence can be wonderfully inventive, or it can be a by-word for routine repetition. Given Terraria to date has been a wild grab-bag of ideas it seems unlikely that Otherworld's take on TD will be perfunctory tedium. TD has been included because "even the best players would not be able to cover and defend the entire map at once (nor does that sound all that much fun)", so this way you get to potentially defend your holdings without having to sprint madly around the place all the damn time.
You can see some of the tower defence stuff in the above trailer, and there are a few more gobbets about new features at this here link. Perhaps most ambitiously, the devs want Otherworld's NPCs to evolve into more than mere merchants, instead offering their own little stories and personalities. There is an overall main storyline too, although apparently you won't be forced into doing anything.
It sounds as though they're aiming for something a little more focused and structured compared to the somewhat manic Terraria, and I'm down with that in theory.
Release is planned for some time this year, but devs Re-Logic are determined that it will be a "full" game at launch. Something of a tall order in this age of paid alphas and early access, but we'll see how it pans out.