Dungelot is such a simple and fun idea that I'm surprised it's the first time we're seeing a game like this on the App Store. It's sort of an abstraction of the standard fantasy-based hack-and-slash dungeon adventure, with just one mechanic: To explore a square on the grid-based dungeon that you're going through, you simply tap it.
That's it -- as squares open up to you, one by one, you'll find loot, health, magic spells and, of course, enemies to battle, along with a series of other surprises. All of the action is completely turn-based (though there are some minigames hidden in the depths), and just like a standard roguelike character, your goal is to get down as far as you can before dying.
The game is great -- it seems to be the project of one industrious Russian developer, and while I experienced a few bugs when the title first released, they seem to be all fixed now. Dungelot is a great and simple game that combines casual and complex with probably more flair than you'd expect. It's US$1.99 on the App Store right now, or there's a free version to try out as well.