I haven't played Magic: The Gathering seriously since junior high. However, now that I think back, I believe it was the art that really drew me in. I'm sure many have a similar appreciation for the quality of the art in this game. If you are one of those people, you might enjoy this article from Ed Grabianowski of io9:
I'd also be interested in seeing any other cards people think should have been added to this list (you should be able to find pictures of all the cards here).