Steam World Dig reminds me of this old Xbox Live Indie game called Miner Dig Deep. They both have the same basic idea. Dig for valuable minerals, return to the surface to sell off your gains, buy new equipment & supplies, repeat. It's a simple concept but it's addictive and entertaining. Steamworld Dig adds to this concept and creates a very nice game.
The goal of the game is to make your way to the bottom of an ancient underground labyrinth. Mostly you'll be digging your way through, however you'll make occasional detours to collect special upgrades. These upgrades include the ability to dig through rocks, fire projectiles, and eventually even double-jump. It's kinda comparable to Metroid, since you can use the upgrades to find better stuff in previous areas. There are also various creatures to deal with. There are a handful of ways to deal with them aside from just using your pickax. This game is really combat-oriented though, just consider it another obstacle in the way of your loot.
Steamworld Dig is a very well-paced game. Aside from monsters, there are also traps and other issues that crop as your scour the depths. There might even be times where you can't find a way back up. Unless you can find a teleporter(or brought one with you), the only alternative will be very costly repairs. Getting crushed by rocks is also a thing that can happen. Really though, this is an easy game. It's very much something that you put in to relax for a bit.
Not to say that this game doesn't have any challenge. There are end-game rankings as well as a variety of ways for you to challenge yourself. Maybe on your next play-through you can skimp on the health upgrades, or devise that perfect route to get through the game as fast as possible. For some one trip through the labyrinth is enough, but Steamworld Dig does offer more, and the good controls and playablity make for welcome return trips.