Another important skill of a game designer is the ability to appreciate the history of the game. One of the reasons I got so into Magic trivia was that I loved looking at all the small details of the old sets. Just as players learn more about the game over time, so too do the designers. If you want to understand how the game ticks, you need to have a good comprehension of the lessons learned from the past.
For this reason, whenever I work on a new set, I always go back and look at any relevant cards from the past. For Mirrodin I looked at not just every artifact but every card that had the word “artifact” in its text box. It was important that I had a good understanding of what we had done with artifacts in the past. As the saying goes (or at least as it's loosely paraphrased) , we advance because we stand on the shoulders of giants. And in Magic, that saying sometimes gets quite literal.