I have found the fact that Grunt lets you write full-featured JavaScript quite useful. In 95% of cases, of course, you want to write your configuration in a declarative, JSON-like form, but I welcome the possibility of having full JS power in the few situations where non-trivial logic is needed. Another advantage, of course, is familiarity: I already know JavaScript.
However, what I really don't like is a declarative data language or some sort of DSL that starts adding some basic variable and control flow features. That's the best way to end up with a tool that's complicated, hard to reason about, and still unexpressive.
So to me, a good configuration language should be either a simple data language (such JSON), OR a simple, powerful, well-known programming language with good data structure literals to encourage a declarative style.