“HFCS is just sugar.”
Highly concentrated sugar actually. The issue is not so much sugar which is not problematic consumed in modest amounts, but the very high concentration of sugar introduced when consuming HFCS products.
The problems associated with eating HFCS are exactly the same as the problems associated with eating any other kind of sugar. If you eat it in high quantity, you will gain weight.
I seem to remember reading somewhere that HFCS is sweeter than regular table sugar, meaning that the same amount of sweetness can be achieved using less of the actual sugar.