They real cool thing about dylan is its objectsystem. Common Lisp has an objectsystem called CLOS (common lisp object system) and it basically is the nonplus ultra of all objectsystems, but CL has the problem that it's kind of mmhh well kind of an unclean design. The Dylan people wanted all the power of CLOS but in a clean new design.
The main thing which is special is that you do everything with multimethods which are outside of the objects. It's really awesome but the benefits are kind of hard to see in the beginning.
The other thing is that dylan has a macrosystem, which works pretty ok. I have not seen a better macrosystem for a language with syntax.
All in all dylan is a really awesome language, it just had a lot of bad luck. Everybody should try to learn some dylan, it's a language with a lot to teach. The OpenDylan People inherited a shitload of code (compiler and libraries ...) and they are working on making it production-ready.