On the subject of being demanding of the listener, I think most instrumental music immediately becomes more demanding than lyrical music. The moment you add text, the musical content takes a back seat and doesn't really need to hold people's attention as much. It has more concrete meaning, is easier to make judgements about, and is grounded in day-to-day human interaction, rather than the abstraction of raw instrumentation.
It's hard to really say that classical music is particularly any more or less demanding than jazz, or some instrumental rock (jam bands, instrumental guitar, etc). And I would say classical opera is a good deal less demanding. Though I would say things with less harmonic and rhythmic complexity are typically less demanding. New Age is largely instrumental, but it's mostly background music, not really meant to demand much attention at all.
The one major exception is instrumental dance music (again, irregardless of genre). Be it EDM, Appalachian Square Dance, or Johann Strauss.