Played it for a few hours so far. Pretty fun, though certainly not a Minecraft clone. It's more of an adventure game with crafting and mining built in. There's a large chunk of things that simply can't be crafted or mined, and are instead obtained via dungeon/treasure hunting. Hell, it even has boss fights. Certainly a LOT more emphasis on combat than in Minecraft.
But yea, both Minecraft and Terreria are great games. One is more for building, the other is more for adventuring. I can honestly see that people bored with the lack of things to do besides build in Minecraft would probably like Terreria more since it's more action oriented.