Considering quite strongly a buy.... How many biomes, monsters, things to make, items etc.?
This game seems less about building and more about exploration, so it needs a lot of these..
I'd check out the Let's Play videos and wikis if you're on the borderline of purchasing.
It's a lot like an endless boulderdash-Roguelike sandbox game played in 2D. It's not quite about slaying monsters as it is that monsters are like creepers---things that can annoy you and prevent your exploration progress.
You're right that the game play is primarily exploration since the main point of the game is exploring to obtain resources for crafting to upgrade equipment. Biomes are more like window dressing and rewards for exploring (either horizontally or vertically). There are vertical biomes (floating islands, hell, underground jungles) and horizontal biomes (a dungeon, an ocean, an area called "Corruption"). The typical biome is really forest, and then there are some small seas/lakes, and some deserts (full of sand which is a falling hazard if you aren't careful digging).
I'd say it's very much like a 2D minecraft except they exploit the 2D platform aspects to the fullest. So if you have a 2D retro fetish (e.g. like jumping,, then you'd probably like the game, but if you prefer 3D then you might want to stay with Minecraft.
Building is secondary but still significant since as a sandbox game you can build absolutely however you want. You can completely redesign the map and use the game worlds as kind of a design your own platformer kit.
Leveling your items for crafting is pretty secondary too. It's basically like copper axe -> iron axe->.....gold axe-> named axe (like a roguelike). There are some special items in the game like swim flippers, snorkels, goggles, grappling hook (very useful), rocket boots, etc but a lot of it is basically sword, axe (weapon/chop trees), hammer (build tool), pick (digging), bow/gun. There's also alchemy and some magic.
All in all its pretty rudimentary right now. Being on Steam, and since they aren't officially supporting modding, I'd expect another few months of development followed by DLC.
At this point seems like it would become repetitive quite fast.
