A white potato is a vegetable -- a starchy root vegetable. They're high in fiber, potassium, vitamin C -- one medium potato fulfills about 10% of your fiber Recommended Daily Allowance (RDA), 20% of your potassium RDA and 45% of your vitamin C RDA. And eat the skin, too, because it adds other nutrients to the mix (iron, calcium, zinc, phosphorus, B vitamins). They're higher in calories that non-starchy vegetable but one medium baked potato has only about 110 calories.
So potatoes can be a "good" carbohydrate -- the problem is usually all the "stuff" that you put in or on potatoes (butter, sour cream, cheese) or how they're prepared (french fries!). And starchy vegetables do raise the body's blood sugar levels (something diabetics especially need to keep in mind).