Lots of good answers in this one.
What makes a good design for me is:
1. Fun (not in the zero-sum way, bonus if it sells a dream)
2. Clean (not too wordy, within the colorpie)
3. Balanced (playable at many different power-levels)
My answer is clear and not named in the thread.
In your opinion, what's the BEST designed card in Magic's history?