What nutrients are concentrated in citrus peels (not juice)?
You can look up nutritional information at the USDA nutrient database. Their entry for orange peel says that 100g of orange peel contains: 97 Calories, various minerals, and a few vitamins. You can compare it raw oranges to see how it differs from the rest of the fruit. You can also look up orange juice, etc.

You can also get some of that information (plus comparison to USDA recommended daily amounts and to other foods) at Wolfram Alpha. You can even have it compare orange peel and orange.

As rumtscho mentions, the pith (white part of the peel) is bitter. The zest (outermost colored part) is very fragrant (and flavorful).

Oranges vary in sourness depending on variety, cultivar, growing conditions, etc.