I LOVE modern. My school puts a lot of emphasis on modern, so these past few years I've really gotten into it. Having a strong background in ballet or jazz really does help, and so does strength and flexibility. You don't need a crazy amount of other, though... seriously, modern is just a lot of fun and can be really beautiful.
Also, it can depend what kind of modern they're teaching. A lot of classes are just freeform, which can be great or can be awful. We do mostly Graham at my school, which is REALLY technique-based - I'm pretty good at Graham because it seems so similar to ballet, which is my area of expertise, lol. We also do a little Horton, which is the main style done at Alvin Ailey. I personally HATE Horton because it feels awful on my body and does funny things to my knees and Achilles tendons, but some people love it.
Okay... done rambling... haha, sorry for the book.
"Sawyer, you're going out a youngster, but you've got to come back a star!"