Also, I'd like to add that I heard (I haven't actually read the literature on the matter, I just watched a lecture, so it may or may not be correct) that the liver is better equipped to metabolize fructose, while the rest of our body deals with glucose (comparing just these 2 hexoses: obviously there are more sugars in our bodies than just these two), so the metabolic waste/stress that results from a high-fructose diet is more likely to accumulate in the liver which has negative effects on the body. This observation does not mean that fructose is bad! The most common context (as mentioned by Ringer and iNow) of fructose in a natural diet is in fruits. The presence of fructose in fruits is not necessarily bad because there is lots of fiber which prevents some of the absorption of the fructose, and because of this, fruits do not cause a noticeable amount of metabolic stress on the liver. However, in a diet where the major source of sugar is a high-fructose corn syrup (which is a mixture of 50% fructose 50% glucose), there is typically not that much fiber and consequently absorption is elevated, leading to an increased amount of fructose metabolized in the liver, and a larger amount of metabolic stress to the liver.
Anyhow, this explanation is what I have heard. It would be great to actually see the data. Someone let me know if any of you have heard something similar!