High Fructose Corn Syrup - Despite the fact that ALL of Europe has banned the use of HFCS, it is still in almost anything you can pick up in America, especially foods targeting children. For 5 non-debatable and comprehensive, yet brief, reasons to never eat HFCS again, check out 5 reasons HFCS will kill you. It's not as dramatic as it sounds, just useful.
Enriched/Refined/Bleached Flour - When flour is refined, all of the nutrients in the original grain are destroyed. Companies then "add" small amounts of nutrients back into the mix and are allowed to use the word "enriched" on the ingredient list. Fiber, An important components that is lost is not added back. Fiber helps to promote fullness, slow digestion, and prevent the rapid, harmful spike in blood sugar that occurs upon the consumption of enriched flour, or white bread.
Hydrogenated or Partially Hydrogenated Oils (trans fats) - "Hydrogenation is the process of heating liquid vegetable oil and combining it with hydrogen gas. Hydrogenated fats or oils are also called trans fats, a term which refers to the changed bonding arrangement of carbon atoms. These man-made fats have no health benefits, and are known to raise cholesterol and contribute to coronary artery disease."  Common foods that contain hydrogenated oils are vegetable shortening, cake frosting, white breads, fast foods, frozen foods, chips, and soups.
MSG or Monosodium Glutamate - This "flavor enhancer" in used openly in lab studies to induce obesity in lab animals so that studies can be conducted on them. MSG is relied on to induce obesity. This ingredient is found in seasoned Pringles, Doritos, and used in food in almost every fast food chain in America. If you're eating whole foods rich in color, you won't need to worry about adding an obesity inducing "flavor enhancer."
Artificial Sweeteners aspartame, saccharin, Sucralose, acesulfame-K - Artificial sweeteners have been proven to promote obesity and cause cancer. Watch out for foods that tout "sugar free" or "zero calorie" claims. Check the labels of these foods and make sure they do not contain any of the above sweeteners or ingredients. Chances are they are replacing the natural calorie-containing sugars with these artificial sweeteners, and the health costs of these are far worse than the few extra calories you are "saving." Artificial sweeteners are far sweeter than natural sugars and they train our mind to prefer, and actually crave, extremely sweet, empty-calorie foods and cause us to shy away from healthier, more filling foods.