Obesity, pain. Humans have a limit to how much fructose the bowel can absorb at any given time. If you max out this limit, the fructose stays in the bowel and causes pain, bloating, and diarrhea. This is not dangerous, however. Just uncomfortable. The real health problem with hfcs is that because of its sugar contents, it is a major contributor to obesity and its consequences such as type ii diabetes.
Depends who's asking. I have done quite a bit of reading on this. Studies in mice show metabolic syndrome (middle-age man syndrome) can be induced with high fructose diets and reversed when it is removed. This is not true of glucose or sucrose. See the article: "research offers insight to how fructose causes obesity and other illness" sciencedaily (feb. 27, 2012) or am j physiol renal physiol. 2006 mar;290(3):f625-31.
Yes. Main culprit for the obesity epidemic.