Godwin's Law rules the land, no matter how apt the comparison, how accurate the simile. If "we're not nearly as bad as the Nazis" is a hair they're happy splitting, then I will be content to stand by, slowly applauding. And laughing until nausea sets in again.
So if each period of history is unique, and one cannot justly compare one to another without opening oneself to charges of ahistoricity (which is, after all, what the Righteous W's now bray), then all historical similes must die. No more Hitlerian or Neronian; no more Draconian or Spartan. Inapt. Not Comparable. Times Are Different.
What solecisms will history undergraduates commit in future years, when asked to describe the atrocities and genocides that will (beyond a shadow of doubt) deface their own times? Will angry red professorial pen strike out words like Bushian and Rovian and Rumsfeldian and scribble in margins, "NO! We're not as bad as that!" Will some future Dick Durbin in some far-off future time have to genuflect abjectly when Righteous W's take wattle-shaking umbrage at his employment of the adjective Guantanamesque?