Magic cards guest-designed by Sirlin, Justin Gary, etc.

A couple of the designers talk about their cards (which had to be designed 2 years in advance):

Edmund McMillen, the "Super Meat Boy" designer:
Designing Cruel Sadist

Notch, the "Minecraft" designer:
Designing Aggressive Mining

I really don't mean to keep bumping this thread, BUT this is by far the best link, and the final one:

Working with Some of the Best Minds in Gaming

This shows each designers original design (before they got heavily tweaked), talks about why the changes were made that were made, and how each card reflects the designer's own games and philosophy.