First of all someone should define what "sugar" is: in non-scientific usage "sugar" = "table sugar" = sucrose. Sucrose is one glucose molecule and one fructose molecule linked together; when it gets into your small intestine the linkage is broken and you get glucose and fructose separately, no different than if you ingested HFCS. In HFCS the gluc/fruc ratio varies, but the most common forms are 55% and 42% fructose so both are more or less the same as sucrose.
Glucose is not a metabolic issue: every cell in your body uses it for energy, in fact almost all forms of life (even bacteria and archaea) can use it as well. Your brain can use ONLY glucose, BTW, it cannot metabolize proteins of fats for energy the way other parts of your body can.
Fructose is a different matter. Evidence has been piling up for years that fructose is involved in the "metabolic syndrome" cluster of disorders, e.g. obesity, type 2 diabetes, cardiovascular disease, etc. Fructose also contributes much more to the formation of inflammatory abdominal fats than other sugars.
Lactose is not a constituent of table sugar. It is composed of glucose and galactose. I have seen some rumblings about potential health problems of galactose but it doesn't seem to be a hot topic. Lactose intolerance can be a problem though, which results from having a mutation in your lactase gene, which codes for the enzyme that splits lactose into its component monosaccharides. The pancreas secretes lactase into the small intestine, but if it doesn't do it's job quick, then your gut flora will start digesting the lactose and producing acids (and CO2), which irritate the intestine causing diarrhea and gas. Taking probiotics or eating predigested dairy products (e.g. yogurt) reduces the problem. But none of these issues overlap with the problems caused by sucrose/HFCS consumption.
Considering that sucrose/HFCS only has two components, one is vital and the other not, I think that alone should serve as a smoking gun as to which one is the problem, if sucrose is causing problems...
Here is a good review article (even back in 2005) on the known medical issues surrounding fructose intake:
http://www.anaturalhealingcenter.com/documents/Thorne/articles/fructose10-4.pdf
Something even more hard-hitting, I think, is a recent video in UCSF's public YouTube channel, by Robert H. Lustig, MD, UCSF Professor of Pediatrics in the Division of Endocrinology -- titled "Sugar: The Bitter Truth". Be prepared for some biochem, but the lecture covers much more ground than that. It provides a very clear argument for considering fructose a toxin -- once fructose hits the liver, the metabolic impact is more or less the same as alcohol -- which explains why chronic alcoholics and heavy consomers of sugar/HFCS wind up with the same health issues down the road: