A question for the specialists...

Whenever anyone asks me what I, IV, V means, I give the easiest example of C, F, G.

I am probably making an assumption, but most Americans here seem to say 'C, F, G7'

As such, all I IV V tunes become mixolydian. Is this a result of early blues music, or just a national taste?

The most 'english sounding' (whatever that means) music that I know never has a flattened seventh.

Any thoughts?

Wolf