From David Dayen at Firedoglake:

We’ll hear this was necessary because of Awlaki’s operational role, and his part played in the Fort Hood massacre and the Christmas Day bombing attempt.  In reality, many experts saw Awlaki as a public figurehead, a man who could speak in front of a video camera and therefore was magnified in the US public’s mind as some sort of leader.  He was a guy with a YouTube account rather than an operational leader of Al Qaeda.

And now that he’s dead from a drone missile strike, it’s too late to wrestle with the question of whether a US citizen should be designated as a candidate for assassination by a murky process with no checks or balances.  The President and leading counter-terrorism officials just decide who should be marked for death, and then robot planes carry it out.  That slope is so slippery I’m assuming the BP Deepwater Horizon well is nearby.

And Glenn Greenwald:

What’s most striking about this is not that the U.S. Government has seized and exercised exactly the power the Fifth Amendment was designed to bar (“No person shall be deprived of life without due process of law”), and did so in a way that almost certainly violates core First Amendment protections (questions that will now never be decided in a court of law). What’s most amazing is that its citizens will not merely refrain from objecting, but will stand and cheer the U.S. Government’s new power to assassinate their fellow citizens, far from any battlefield, literally without a shred of due process from the U.S. Government.

And frequent commenter myiq2xu:

The President of the United States ordered the killing of an American citizen without any trial or due process of law.

How do we know he’s guilty?  Because THE GOVERNMENT SAYS SO!!!

These are legitimate concerns, but made as to the wrong person.

Bad facts make bad law, and Anwar al-Awlaki was a bad fact.

Update:  Kenneth Anderson at Volokh Conspiracy has background on the al-Awlaki legal case in which the ACLU unsuccessfully tried to get an injunction against his targeted killing.