HFCS has come under a lot of scrutiny and blame as it came into vogue as an economically driven substitute for ordinary cane and beet sugar (i.e. sucrose) about the same time as people started getting fat at an increasing rate. Sucrose, dating way back in modern history, still somehow didn’t provoke much of an obesity problem of itself.
The hypothesis is that it is fructose that is a problem, regardless of the source (HFCS, sucrose, etc.).
HFCS is most likely not more “evil” than sucrose in metabolic terms, although it might have slightly increased fructose concentrations in soda, etc.
However, it is cheaper and more ubiquitous than sucrose (partially because of federal corn subsidies and sugar tariffs).
Refined sugar used to be a rare and expensive commodity, until the early 20:th century. On average, people consumed perhaps 16-24 grams per day of fructose before WW II.
In the late 70-ies, according to the USDA (NFCS), americans consumed 37 grams per day of fructose.
In 1994, fructose consumption had increased to 54,7 grams per day (NHANES III).
In certain important subgroups, consumption has exploded. US adolescents on average consume about 73 grams / day of fructose.
As the fructose hypothesis is based on the problem being a threshold effect, I.e. the liver getting overwhelmed in a manner similar to what happens in chronic alcoholics (not a coincidence, ethanol is fermented sugar), the effect of fructose on NAFLD and obsesity is most likely strongly non-linear. A little is completely harmless, a lot is dangerous.
I personally believe there might be more going on than just the fructose, in terms of disrupting people´s appetite control systems, but I am pretty convinced that fructose consumption is an important piece of the puzzle.