I'd say that one requirement is to forget some of the things Java taught you (us). You'll need the willingness to approach OO from a different angle, because Ruby feels decidely "not" Java at times. If you don't allow yourself to "let go of Java" for a time, you'll end up writing Java code with Ruby syntax... a travesty if there ever was one!
Why's Poignant Guide to Ruby was the first real look I had at the language, a great (if not a little strange) introduction. Glad that it's still available.