Over on Man Bytes Blog, there's a post about dice.
I like dice, but I'm coming to love cards more. This isn't an attempt to convince him to switch from dice to cards, just an attempt to show you some interesting features of cards.
Of lesser interest is the many different ways to interpret cards. A die is simply a flat number. A card, however, is a number, or a face card, or a suit, or a poker hand - anything you want, with a complexity level way higher than dice. Yet it is still just as easy to read and considerably less likely to bounce off the table.
The interesting feature about cards is that they come from a deck. If you're using a standard deck, once you've drawn the queen of hearts, there's no more tart-makin' ladies to be drawn.
My philosophy when it comes to in-game challenges is one of puzzle optimization.
I do not do the "roll to live" game style. Nor do I do the "figure it out or die" game style. Instead, I run with attrition. Even as you gain experience and property, every action, every encounter weakens you.
Damage heals slowly. Mana regenerates slowly. Ammo goes away, and often must be replaced with a completely different kind of weapon. Sure, they also get more powerful as time goes on. The difficulty for the players is in doing the latter faster than they do the former.
Cards are built for this kind of hoobie-jooby. A deck of cards can represent energy, stockpiles, hordes of enemies. When it runs out, it runs out!
The key here is to allow players to mix cards back into their deck in certain situations, so they can choose to stack their deck. For example, they can choose to spend good cards early and stack the deck against themselves, or spend shitty cards early at stack the deck for themselves.
And, of course, allowing them to change which cards are in their deck - often a more valuable option than gaining a new skill.
The end result is that you end up with a tangible stack of character, specially made for an attrition-based game. You end up with methods of play that are much deeper than die-based systems without having many more rules.
Moreover, by allowing players to read cards in different ways by using different types of play, you allow players to stack the deck against themselves, then switch methodologies such that the deck is no longer stacked. IE, they can read for suit instead of number. This further enhances the RP and customization: for example, someone who starts precise and careful, but gets emotional as he gets tired.
That's why I like cards. Every action you take with a deck of cards has a lasting effect, no matter what the outcome of the action. Every player will approach it differently. Every play style is an opportunity to RP.