As it stands now, Terraria is starting to strike me as the kind of game you love intensely for a few incredibly long play sessions...and then you hit the peak. Like, you make your fort, you kill the bosses, you build the best loot and....another play through amounts to doing the exact same thing.
Not that that's bad. But I dunno if the game will have the randomness and variety that you might get out of say, DF or a rogue-like. While I'm still really pumped for the game, some multiplayer withstanding, I think it's going to be a game I binge on for a good long while, then struggle to find a reason to come back to it. Maybe, maybe not. They may do things in development where you can't see everything, get everything and do everything in one game, with more biomes, items, bosses and areas that aren't all present at once. Games like that keep me coming back time and again, especially single player.
Put another way, I wonder if Terraria will end up somewhat like Magicka where it's a blast until the very end, but then you're pretty much "done." I'm still a guaranteed sale (Shut up and take my money already guys), but I wonder on average how long it takes to "complete" a full game. There really isn't quite enough going on in terms of the world or the NPCs to really create persistence. The corruption is a nice start, but there needs to be more.
And in a totally unrelated line of thought, what's the plan for gems I wonder? Redigit and Blue treat them as purely ornamental, but they gotta have some more planned for them than that. I wonder if you'll be able to set gems in either furniture or gear.