Hi everyone, I have been a lurker since time immemorial, and I finally have something to post.  This could be a long one

I am building a game that uses ordinary playing cards for character creation, conflict resolution, and campaign building.  It has a definite fantasy flavor, as each suit matches up with the old familiar "Four Elements".  As I am building this, I had a few queries for the learned and experienced out there...

1. Are there many games out there that use playing cards that I could, er, mine for inspiration?

2. Are there any common pitfalls associated with using playing cards as opposed to other items (like dice!)? 

Here is a very quick overview of how things work.  Currently, I am working on the mechanical "skeleton" of the thing and I have yet to begin on the campaign or world specific stuff.  For your reference, this game has been influenced by Fudge, Universalis, and a game my friend made in college.

What you need:
Decks of cards (Typically two, with different colored backs.  One for in-game play, the other for in-between session development).
Some willing victims.

Four Elements and Four Suits:
Air...Spades
Water...Diamonds
Earth...Clubs
Fire...Hearts

Character Creation:
The character components are: Statistics(Body, Mind, and, Soul), Attributes (Earth, Air, Water, Fire), Special Disciplines, Advantages, Disadvantages.
Each player draws 10 cards.  Cards of values A through 10 are sorted according to suits.  Face cards are sorted according to face.  Jokers are set aside.
The face cards you sorted will be your Statistics.  Every King gives your character a +1 for Body.  This works similarly for Queens and Mind, and Jacks with Soul.
For the A through 10 cards you sorted into suits, you choose Attributes that belong to each element.  These can be professions, skills, maneuvers, or other descriptors.  A few guidelines need to be followed though.  Earth Attributes are things of solidity and substance like Blacksmithing, Massive Body, Incredible Strength.  Fire Attributes carry energy and passion like Sculptor, Intense Disposition, Accomplished Orator.  Air Attributes are things of communication and thought like Skilled Codebreaker, Tactician, Diplomatic.  Water Attributes are things with quickness, flow, or sensitivity like Affinity with Plants, Fencing, Detect Magic Auras.  The choices need to be reasonable and approved by the GM.
If a Joker is drawn, a Special Discipline may be chosen at the game's beginning.  This could be Magic-User, Ninjitsu, Psychic Ability, etc.
Advantages and Disadvantages are created entirely by the player, with GM approval.  The main guidelines being: Advantages cannot dominate and it isn't a Disadvantage if it does not create a hindrance of some kind.

Example: Flindlesticks Underroot (Halfling)
I drew 3 Water, 2 Air, 2 Earth, 1 Fire, a King and a Queen.
Body: +1
Mind: +1
Soul: +0
Incredibly Sneaky (Water) +2 (I used two Water cards for this)
Alertness (Water) +1
Fast-Talker (Air) +1
Knows when to turn on the Charm (Air) +1
Food Enthusiast (Fire and 'cuz he's a Halfling) +1
Barroom Brawler (Earth) +1
Stout Build (Earth) +1
Advantage: Many minor contacts
Disadvantage: Horrifyingly greedy

Basic Mechanics:
A through 10 = 1 through 10
J = Automatic failure
Q = Automatic draw
K = Automatic success

There are two kinds of events that require resolutions: Contests and Tasks.
A Contest consists of two characters that are opposing one another.  Each player (or GM) draws a card.  Statistic bonuses are added (if applicable) and Attribute bonuses are added (if applicable).  If the card a player draws matches the element of the Attribute he or she is employing, an additional +1 is gained.
Tasks occur when a character is acting against something passive or wholly inactive.  Lockpicking, jumping a chasm, etc.  The GM draws a random card.  That card's value becomes the target that must be met or exceeded.  The GM also assigns a difficulty modifier between -3 (fiendishly difficult) and +3 (incredibly easy).  Additional modifiers function similarly to that of Contests (Statistics, Attributes, etc.).

There is more, elaborations on determining degree of success/failure, group campaign development, between session development, and other stuff, but I don't want to bore everyone with my first post.

Geoff

Quick response to your two questions, then I'll go back and comment on the rest of your post when I have more time.

1. Yes, there are some games you can mine for ideas. Check out Dust Devils and my 24-hour game submission Trapped!

2. The most common pitfall is just using the cards as a randomizer replacement for dice. Dice commonly exploit two or three properties number of sides, value of the die, number of dice in your pool. Cards offer more options: value of the card, suit of the card, matching cards, poker/rummy combinations, number of cards in your hand, face up/face down, drawing random cards, playing known cards from a hand. You can do a bunch of these things with dice, but they are easier to do with cards and the operations are more intuitive with cards due to most players' experience with card games.

Trapped!, for example, gives each player a replenishing hand of cards to use in challenges. Cards are played and totalled for a score. Traits are rated as cards (Sexy: 5 of Hearts, Lucky: Q of Clubs) and act as virtual cards that are always in your hand, even though you don't get a card for it. Suits of cards represent relationships on characters (Hearts: love & respect, Clubs: hatred), and theme in events (Hearts: Relationship, Clubs: Violence), and mode in actions (Hearts: appeal for aid, Clubs: intimidate or force).

I could have done more with the cards, but at least I touched on several of the properties: card value, suit, drawing random cards for character generation, playing known cards from your hand, hiding cards from other players. I even did an "opposition" thing where Hearts and Spades are opposed and Clubs and Diamonds are opposed and this affects who you can help and hinder.

Hi, Geoff! Welcome to (posting on) The Forge!

My superhero game, With Great Power... uses playing cards for resolution (link in .sig). Also card-based are Castle Falkenstein (long out of print), Psychosis (same), and Prime Time Adventures, and Nine Worlds.

As Adam noted, there's a lot of room for development in card usage. At the moment, it looks like your resolution uses cards as a straightforward randomizer. And from the looks of things, a VERY random one at that! I mean, your base abilities range from 0 to 2, but your card values go from 1 to 10.

This is not a bad thing, depending on what your game is about. If it's a game about the capricious nature of fortune and luck, it's perfect. If it's a game about relying on yourself and trusting your friends' abilities ... there could be problems.

So, Geoff, what's your game about? Only knowing that can we help you identify what's a help and what's a hindrance about cards.

As an additional source of inspiration, possibly:  Everway used tarot-like cards rather than playing cards, but character abilities were "elemental" in a way similar to what you're proposing, if I recall correctly.  It may also be worth it to map the numeric values of the cards to meanings just as suits are mapped, so that e.g. the Seven of Hearts can be read as "Lucky in Love" (because 7 means luck, good fortune, or wealth and Hearts means Fire means Passion means Love).

Personally, I have a personal bias against card-based systems (except for Nine Worlds) for the simple reason that they're usually quite difficult to play over IRC, which is my normal venue these days.

I have never played over IRC, so I never really considered that medium when I was thinking things up.  As I work on this, a problem keeps popping up in my head.  When you roll dice, the probabilities for getting different number remain consistent.  When you play with cards, the probabilities are constantly changing.  So, do I reshuffle after each "scene" or "encounter", or do we play through the deck, taking what comes forth?

What is my game about? 
Here is the extremely short version...
A cataclysm of supernatural origin has destroyed a world.  The magic that used to bind the world has been massively altered.  The world is now a bunch of floating chunks that are bordered by a strange haze.  Fortunately, some folks were prepared and engineered these bio-mechanical devices known only as "green monoliths".  These green monoliths surround the most habitable chunks, called Nexuses.  Unfortunately, the byproduct of the cataclysm is that the psychic backlash caused almost every single memory out of the inhabitant's minds.  Fortunately again, these same well-prepared souls stashed knowledge away in vaults that are located in these Nexuses.  People find certain bits of this knowledge and learn how to maintain the monoliths, they also learn how to travel to the other bits of lands called Shards.  These are unstable pieces of land that contain many of the resources needed to maintain society in a Nexus.  Also, a Nexus experiences no weather and exists in a permanent twilight.  Shards experience weather and night/day.
A brotherhood called the Grey Men has arisen and they have the skills needed to travel to Shards.  They lack one thing, the ability to predict the ever changing pathways between Nexuses and Shards.  This is where dreams come in.  Dreamers get "instructions" to meet dream guides in the real world.  These are usually in animal form and can help players navigate between different lands.  The guides are basically the tethers for the adventurers.  There is much more, but I will save that for later.
So, the elements that I need to work into my system are:
1. The power of dreams to aid and guide
2. The need for magic as a means of transportation and safety
3. The existence of living machines (non-sentient)
4. Building a society from scratch
5. Intrigue...are the Grey Men sharing all the stuff they find?
6. Adventure...goin' to Shards, meeting folks from other nexuses.

in Nine Worlds, the player chooses when he reshuffles his deck.