Mark Rosewater is the Head Designer of Magic, and has probably written more about designing cards than anyone else in the world in his Making Magic column.
Design 101 describes the common mistakes most novice designers make.
Mistake 1: The Card Is Too Complicated
Mistake 2: The Abilities on the Card Have No Synergy
Mistake 3: The Card Ignores Basic Design Rules of Magic
Mistake 4: The Card Doesn’t Work Within the Rules
Mistake 5: The Card Is Undercosted, Overpowered, or Simply “Bah-roken”
Design 102 contains the things Mark feels Magic designers could do to get better.
1: Know Magic History
2: Play Magic
3: Design a Lot of Cards
4: Know What You Want
5: Play With the Cards
6: Have Other People Play With the Cards
7: Give a Set Time to Breathe
Design 103 has some more advanced mistakes.
Mistake 1: Making the Audience Do Something They Don’t Want To Do
Mistake 2: Making the Audience Do Unnecessary Work
Mistake 3: Don’t Put Things They Care About Out of Their Control
Mistake 4: You Force The Players hand Too Much
Mistake 5: Making Cards Match The Wrong Audience
Design 104 takes Dr. Roger von Oech’s “creator roles” to explore four more mistakes.
The Explorer’s Mistake: “It Hasn’t Been Done” Is Not a Reason to Do Something
The Artist’s Mistake: You Try to Be Too Literal
The Judge’s Mistake: The Card is Awesome but Not For This Set
The Warrior’s Mistake: You Can’t Fight For Everything
His Nuts and Bolts series go into explicit details about designing a set.
Rosewater also applied Dieter Ram’s Ten Principles for Good Design to Magic.
1: Good design is innovative.
2: Good design makes a product useful.
3: Good design is aesthetic.
4: Good design helps us to understand a product.
5: Good design is unobtrusive.
6: Good design is honest.
7: Good design is durable.
8: Good design is consequent to the last detail.
9: Good design is concerned with the environment.
1: Good design is as little design as possible.
Finally, these two articles cover issues of complexity, especially in commons.