In the countless fawning eulogies for the late Robert Drinan heard on NPR today, another unspoken word-choice media guideline made itself apparent to me. When a thing somewhere on the right is threatening to shake things up, it's almost always "controversial". When a similarly disruptive thing comes from the left, it is often "provocative". Hence, Drinan was quite provocative during his time in office, especially regarding the Vietnam War and gunning for Nixon (I counted "provocative" four times throughout the day but didn't hear "controversial" once). Meanwhile, Bush's argument that failing to support the troop surge will harm the troops is "controversial" (keeping in mind that most of the media errantly categorizes the Bush neocons as rightists).
It's not as tight as the use of "moderate" to describe a liberal Republican while "conservative" is used to describe a conservative Democrat, or as certain as the employment of the adjective "vibrant" to portay a locale as a desirable place to be when there is little that actually makes it a desirable place to be, but it's not a bad generality.
For example, a Google search for "Charles Murray controversial" turns up 956,000 hits; "Charles Murray provocative" turns up 318,000. For "Michael Moore controversial" we get 1,180,000 hits; "Michael Moore provocative" delivers 748,000. Moore is thus over 90% more likely to get "provocative" in place of "controversial" relative to Charles Murray. Using "controversial/provocative liberal/conservative" sees "liberal" as nearly 1,100% more likely to get "provocative" relative to "controversial" as compared to "conservative"--and the web isn't as left-leaning as 'mainstream' media are. I suspect a LexisNexis search will reveal an even more pronounced disparity.