I believe all of the factors above, depending on who are talking about, are contributing to America being overweight. I think kids are eating too much sugar/processed food/garbage food. I think adults eat too much in general, especially when most of us have office jobs. I think Americans tend to overeat unhealthy meats (like hamburger and fatty red meat).

That said, I think other elements of our lifestyle combined with the above are what really do the damage. As you mention in your poll caveat, sedentary lifestyle is a problem. Even for someone like me working out 2 hours a day most days, I often sit the entire rest of the day. No matter how many calories I burn working out, that sitting the rest of the day is problematic. We also don't get enough sleep (sleep levels are show to effect hormones which are shown to have major impact on weight). Many have significant levels of stress (again, hormones).

I think we need to eat better, move more, get better sleep, and change lifestyles to relieve some stress to have a real impact.