Yikes.

I tasted a drink made from unroasted green coffee beans a few years back, when I was talking with the master roaster at a small coffee supplier back home in Brisbane (Merlo). The drink was unpleasantly acidic and kinda grassy, more like tea than coffee. Don't know how anyone could drink that stuff and enjoy it. I think it's chlorogenic acid... robusta beans are higher in this, which is why some people get an upset stomach from drinking cheap coffee.

It's my understanding that the roasting process helps transform the unpleasant acids into more palatable acids. Keep the roast light and you'll get more of the origin flavour; roast it dark and the acidity breaks down further making it better for espresso use. Using too light a roast for espresso can give a very acidic drink which isn't all that pleasant. I tasted a heap of different roasts at this coffee place to find the one I liked (all short blacks... I was buzzing!!!), and for espresso the darker roasts were certainly more pleasant to drink.