How to add flavor to potato soup (using minimally processed ingredients)?
First, cook your potatoes in stock, not water.  I prefer chicken stock but you can use vegetable stock if you prefer.  Make sure you use only enough stock to barely cover the potatoes, and cook them with the lid off.  When the potatoes are done, add milk or cream until you achieve a nice, creamy consistency.  If necessary, you can thicken it by mashing some of the potatoes against the side of the pan.  You can also add some grated cheese.  I would use a hard cheese like Parmigiano or Romano.  Now just season to taste.  With a simple soup like potato, I would stick to salt and pepper only.  If you do use other spices, be careful because they will easily overwhelm the delicate flavor.