Terraria. It's basically Minecraft in two dimensions.
Pros: Freedom and Exploration
You get to run around, digging and exploring, or building as high as you want into the air. There are a lot of things to find all around you. The items are neat, and the interface works well. Thankfully, you don't have to figure out recipes, it just lets you know what you can craft with what's in your inventory.
I particularly like messing with water and lava. Water is the ultimate safety device; you can ride waterfalls down any hole, and it makes a safe platform of obsidian on lava. You can jump higher in water too, so you can "ride" waterfalls up to places that you couldn't normally jump to. I just wish water had some effect on your opponents.
The enemies are terrible. There are three kinds of enemies, and they're all stupid.
The default enemy walks at you until one of you dies. If you're in a small area with them, you can get bounced around by the damage knockback, which prevents you from doing anything at all.
Then there's the enemies that teleport near you and shoot a projectile that goes through walls, so they can shoot you, but you can't shoot back.
Then there are the enemies that "fly" through the ground. Basically all you can do is attack as they get near you, trading damage until one of you dies. Once you get to the biggest ones, it's even odds which it will be, and they just keep coming.
The main problem with all of these opponents is their spawn rate. When enemies spawn as fast as you can kill them, and shoot magic shots at you from another room, it gets old extremely quick. Even the normal enemies can overwhelm you with numbers when they start dropping down from above, and you forget to seal yourself against them.