Discounts for All Cart Items cannot exempt any particular products. However, Buy Now buttons would not apply such a discount because they bypass the cart and take the buyer directly to instant checkout for just one item at a time, but that also means they would not be able to apply any tax/shipping calculation.
Discounts for specific products can only take a flat amount off the per-unit price of the product. However, you can use the same Discount Code for several product-specific discounts (just make sure you give each one a unique Discount Name), so a buyer could enter that one code in their cart to apply a discount to each item in the cart which has a discount using that code. Automatic volume discounts are even easier; just leave the Discount Code blank and set the minimum item total or quantity for each product to qualify for the discount -- note that for product-specific discounts, the total/qty. threshold settings only apply to that item, not the whole cart order.