For those who aren't familiar, Four Against Darkness is a solo pen-and-paper dungeon crawler. You control four characters of various classes at a time. You roll dice, a table shows you which dungeon room or corridor to draw, then you roll again for its contents, etc. In the base game, all combat and all actions are resolved with a simple d6.
Normally, I would balk at this. A d6 simply doesn't permit sufficient resolution. It leaves little room for depth of mechanics. And to be fair, the base game isn't really all that interesting (to me) because of its lack of depth.
But - and here's the thing - there's so much extra content you can get for this game. Adventures, new classes, new mechanics (some of which involves other dice than the d6 although the base mechanic of the game is always based on the d6 as far as I'm aware), etc. For example, the excellent Treacheries of the Troublesome Towns supplement (which comes in two books) adds the ability to enter towns where you will meet all sorts of characters. There are even options for romance, having children, buying a house, and becoming the mayor of the town - there's so much that can happen. And that's just one of the (around 50, I think?) supplements that are out there.
So, in summary, I find myself fascinated by this game despite its lack of complex mechanics, simply because there is so much content for it. If you want, you could think of it as not very deep (read: simple mechanics), but very wide (read: lots of available content).
What are your thoughts on games like this?