In general, many of Nostradamus' prophecies include 16th-century French terms that aren't clear to most modern interpreters. Particular words could be interpreted in any number of ways, and they can be twisted easily to fit an actual event. In Nostradamus' time, for example, "Hister" referred to a geographical region near the Danube river. Most likely, skeptics argue, Nostradamus was referring to this area, not to a person. (Hitler was in fact born near the Danube river, so many believers actually embrace this interpretation).
The most compelling argument against Nostradamus' powers is that his apparent "hits" are the result of random chance and creative interpretation. There are about a thousand quatrains, most containing more than one prediction and all but a few described in vague, obscure terms. Over the course of hundreds of years, it's certainly possible that some events would line up with some predictions, simply by coincidence.
In fact, Nostradamus may have phrased his prophecies with exactly this in mind. Most quatrains refer to deaths, wars or natural disasters, events that are sure to occur again and again throughout history.
Nostradamus' esoteric style also increases the chances of a perceived hit. His metaphorical writing highlights general relationships and conflicts, not specific details. People, or possibly nations, are described as animals; major figures are referred to by their attributes. The quatrain above, for example, refers to "beasts ferocious with hunger," "the great one," and "a cage of iron," all general terms with metaphorical elements. This imprecise language does lend itself well to subjective interpretation -- when the exact meaning is unclear, it's easy to plug in one's own experiences to reach some sort of understanding.
This is a lot like modern horoscopes. Horoscopes typically detail things a wide range of people experience regularly, such as "conflicts at work," "happiness in relationships" or "exciting new changes." Chances are, these predictions will line up with your life, at least some of the time.
In the next section, we'll examine the most recent burst of controversy surrounding Nostradamus -- debate over his possible prediction of the September 11 attack on the United States.