I disagree, my favorite settings always had a very strong central theme. MY favorite DnD setting for instance was Dark Sun. But all of their most popular had clear central themes: Ravenloft, Dragonlance, Forgotton Realms, etc.

I think that without a good strong theme our creativity will be lost and directionless. We need a medium in order for our creativity to clarify and age into something wonderful and that medium is going to be the theme.