From my perspective there are three things that make me love EM's
1) I grew up with them. So they take me back to the happy place right quick. The Artwork, the sounds of the game starting up, the hypnotic paths the ball would take across the PF at slower/spinning trajectories. Something you don't see too often in newer and faster games.
2) The skills needed to play them. This is why many pinheads who stick to newer, faster designs will stay away from EMs. You have to be able to adapt and work the game differently than playing a modern SS/DMD deck. Watch the highest level players play both kinds of games, they can do it all and many times will pull a save on a new stern that many SS/DMD players would not even consider as the trick was pulled out of the EM hat.
3) I take a good look at what designers of the past were doing and how their work influenced the games being produced today. For me the big three were Ed Krynski, Steve Kordek and Ted Zale. Ed was the king of his day. The sheer amount of games he produced (and many were heavily borrowed by other designers or were completed in multiple forms, take El Dorado for example). Steve Kordek while not as prolific or as celebrated as Krynski, did some fantastic work in the early 60's and was the overseer of Williams and their stable of talent to the end. I don't know of any designer that worked with him that didn't appreciate the wealth of knowledge that he brought to the second golden age of pinball. But for me... Ted Zale is the MAN! There are others who did it before him, but the asymmetrical designs of the 60's through the early 70's had more interesting shots and "flow" than the works of either Master K or Centurion K. Add zipper flippers and multiball. Absolutely incredible in a gazillion different ways. Talking to Bill Manke over at Boxwood pinball, the name that comes up most often and the one that I see the most in his designs are influenced by... Master Zale!
Ok, there are more than three reasons... but these are three REALLY good ones!