I agree with very little of what Michael Lind writes, but he has a fascinating piece in the Washington Post on Hitler analogies:

What is at issue here is a matter of moral intelligence, not just good taste or historical accuracy. This kind of casual and unreflecting use of the Hitler smear trivializes both Hitler and the radical evil of the Holocaust.
...The Holocaust cannot reasonably be assimilated to other historical events and trends. The mass death in Cambodia under the communist regime of Pol Pot was not an episode of "autogenocide" comparable to the Holocaust; most of the victims died of a famine caused by socialist agricultural policies, which produced the same result in Mao Zedong's China and Josef Stalin's Soviet Union. The mass executions of political opponents and "class enemies" in Cambodia and other communist states were monstrous crimes, but of a kind all too familiar from the history of dictatorships and revolutions. Nor was the ethnic cleansing of Albanian Kosovars by Serbia comparable to the Holocaust. While the Serbs carried out mass executions of military-age men and mass rapes of women, they aimed to deport, not kill, most of the Albanian population. The Nazis, by contrast, sought to extinguish entire categories of people.
Common sense is missing altogether when the plagues that decimated American Indian populations after their contact with Europeans are called a "Columbian holocaust." Conquerors and traders from Europe exploited and enslaved native Americans, but they cannot be held morally culpable for spreading Old World diseases by sneezing. If they could, then Americans suffering from AIDS and West Nile virus, diseases which spread from Africa, could be called victims of an African attempt at genocide in North America.

I agree, up to a point. Lind is correct to note the influence of early 20th-century theories of eugenics on the Nazis, but he argues:

Even if there had been no Jews in Germany or German-occupied Europe, there would have been a Holocaust of some kind -- the planned, putatively "scientific" extermination of so-called "dysgenic" groups. Stigmatized by pseudoscience as literal "subhumans," homosexuals, the mentally and physically handicapped, and ethnic minorities such as Jews and Gypsies could be exterminated like animals, using methods like those used in industrial agriculture -- the cattle car, the slaughterhouse and Zyklon B, an insecticide used against crop-destroying pests.

Perhaps, but (a) it would've been on a totally different scale, and (b) Lind fails to appreciate the centrality of anti-Semitism to the Nazis program. At most, the eugenics component provided a framework; the animating principle was anti-Semitism.
Lind concludes:

It follows from all this that there should be an absolute ban on Hitler analogies in every sphere of society and every form of partisan rhetoric. Hitler should not be revived in Baghdad, or the White House, or Denver, or the Maryland suburbs, or on the "Today" show. Hitler should be left in Hell, where he belongs.

Sounds good. But the arguments prove too much. Used intelligently (and I'll stipulate that it usually isn't, including most of the examples Lind cites), the Hitler example is: (a) a useful reminder that world-threatening evil does and can exist if we are not careful, and (b) provides a useful standard for inspiring action against lesser horrors. Not for lack of trying, Saddam may not equal the depravity of Hitler. But, as Quentin Tarantino, (of all people) might say, it's "not the same thing, [but] the same ballpark."
UPDATE: Jonah Goldberg has more on the subject.