Is it the on-air "willing[ness] to prettify her image sufficiently to endear her to male viewers"?
Is it that she "makes no effort to hide her casual, butch off-screen appearance, either"?
Is it that she defies classification by being at once butch and femme at the same time?
The reason Rachel Maddow is so hot is because, wherever she goes, she is the smartest person in the room. This country had George W. Bush for president for 8 years, and the likes of Sean "More hair than brains" Hannity are still on the air with viewers. The smart people in this country are sick and tired of morons. Sick of them. Rachel Maddow's sexiness has nothing to do with her physical beauty (or the gendered standards of beauty to which she does or does not conform depending on whether she's doing her own show or Jay Leno's), and everything to do with the fact that she is a Rhodes Scholar, with more brains than pretty much anyone else on the teevee (and I'm not just talking cable), that she's a good interviewer who makes an effort to always be polite to her guests even if they are being douchebags; and then there's that sparkling wit.
So, the sexiness of Rachel Maddow does not make thousands or perhaps millions of liberal nerds like myself want to sleep with her, but rather have drinks with her and talk politics, economics, and other nerdy things that nerdy liberals do. I can honestly say I have no sexual feelings toward Maddow (but I may be in the minority on that one), but the amount of respect I have for her as a commentator and intellectual makes me see her as thoroughly sexy.
And I believe that that is an awesome trend. (I also hope that all of this attention and flattery doesn't make her uncomfortable... I'd never get to have drinks with her if I made her uncomfortable.)
Keith Olbermann on the other hand...