Grant Imahara visited McDonald's fry factory and found out what they are made of. Keep in mind this video series was made by McDonald's and I don't know if McDonald's had a final say on what went into it, but it's still interesting to see how these fries are made and how many ingredients are used to make them: 19 (really, 14, since some of the ingredients are used twice during the process).
Last night, my wife roasted potato slices and the only ingredients she used were potatoes, olive oil, and salt. They tasted great.
UPDATE: See my post comparing the ingredients used to make fries in the US and the UK.