It’s not that iceberg lettuce is bad for you; it’s that other lettuces are better.  Compare the nutrients in a head of iceberg lettuce to a head of romaine.  Notice that the romaine has much more fiber, protein, vitamins A and C, and the minerals calcium and iron.  If you click on “see more” extended nutritional details, you'll see that romaine has more potassium, magnesium, selenium, and other nutrients too.  In general, darker green vegetables are more nutritious.