Terraria released on May 16 of 2011. Minecraft released June 13 of 2009. Almost two years apart, and even though comparisons are made, these two games were very different.
Minecraft 1.8, "The Adventure Update", promises towns with NPC's, certain objectives, randomly generated dungeons, actual reasons for fighting enemies, critical hits and further expansion in combat and so much more. Those are mostly things that Terraria did out the gate. Using the fact that it had dungeons, loot, a central town, and a goal in mind towards the end-game was what differentiated it from Notch's beast. If Minecraft successfully adds these elements in the next update, Blue's 2D Experiment in the "Crafting" genre has no reason for being popular. It would be a different world in a different perspective with some different loot. But it would decidedly be a Minecraft clone anyways, and down the road, most will forget that Terraria was the first one to add focus in to the incredibly popular experience that is, well, crafting better pickaxes to mine more.
Minecraft is a phenomenon. It creates a vicious cycle. You build a house to hide while you mine. You mine to build a new house. You build a new house to explore a new mine. See where this is going? Well, millions of people also see it, but that doesn't stop them from playing almost obsessively, and even more so in the long days of the summer. Terraria on the other hand, while containing the same basic trappings of Minecraft, is based around loot and exploration in a 2D world that is much more manageable. You control your surroundings, and you know where the edges of the world are, and your entire goal is reaching those limits to obtain the best loot in the game in the higher level zones, it just so happens that to obtain some of that loot, you must dig and mine materials to craft some of the essentials.
So now you take that formula and apply it to Minecraft. You add towns around the world. You add dungeons with loot, you add bosses, you add, essentially, an objective that is outside the vicious cycle. You add an end. Something that Terraria has. Once you have the best loot and built an awesome castle, you don't have anything else to do, except do it again in multiplayer. But at least it's not building another house for even more mining. And this idea is very similar to a project that's generating some buzz called Cube World.
The whole idea behind the still-very-early Cube World, is Zelda+Minecraft. It sounds awesome. You get crafting, building, exploration, mixed with items, dungeons with specific tile-sets, bosses and different monsters, all wrapped in one pretty block-based package. Something that it seems the other two main contenders were heading towards anyways. But it really does make you think. Adventures in Minecraft sound awesome, but the premise itself is enough to let your imagination run wild. With obvious inspirations from Zelda, Metroidvanias, and RPG's all popping up in the genre, how improbable is it that we end up with a sandbox that allows for Water Temple's, and flying cities, and amazing loot in randomly generated towns so detailed they seem unique. How long until it's no longer the vicious cycle, and we simply integrate it in to a quest, a purpose...an adventure?
We can't deny it any longer. It's popular. People love it, and they demonstrate it. They buy the products. They spend endless hours in them, and they get excited every time an update shows up. Where will these updates end? When will they end? We can only dream, and those of us that play these games (And love them) can only speculate on what the next version of our desktop fantasies will include. And now that Notch has decided to include mod support in the beta, the community will have even bigger access, with limitless potential. We don't know where this will go, but we can only hope that everything solves itself out for the better, and that there's room in the genre for experimentation.
For now, we can only wait. And there's no better way to let days go by, than playing some Minecraft.