The sane response of people who are actually living in the world to infuriating things is to be infuriated. So being called angry can be taken as a huge compliment, since it means you are sane and you aren’t a sociopath and you actually love life and hate that others are trying to fuck things up.
The “angry” frame is a no-win situation for the Democrats, because a lack of anger means (according the mainstream conservative media) that they’re not energized enough to win; an excess of anger means that they’re unhinged, unstable, and unelectable. The manufactured “Dean Scream” is the most obvious example, but there’s a double standard in place that never allows the “too angry” charge to work against the GOP. Yellin’ Zell Miller wasn’t criticized for his vein-popping display of histrionics at the 2004 GOP convention, and the ever-infuriated Christianist preachers receive respectful hearings throughout the media.
I await a companion piece from WaPo examining the verbal vitriol emanating from right-wing websites; FreeRepublic would be a good starting point, although there are many others.