We know that Anwar al-Awlaki (and Samir Khan) are noxious propagandists who are obviously guilty of incitement to murder. We know this because of their public writings and videos. Is that enough to warrant assassination?
I refuse to accept the word of any member of the Obama administration that they are worse than that. When any member of the administration shows me evidence that they are, then I will consider that they are. But the stakes of killing an American citizen on the say-so of the government are, in my non-lawyer perspectively, simply the gravest of all Constitutional questions.
Something must guard against President Whomever saying, "Oh, yeah, that guy's a dangerous terrorist. Order me up a double-tap." There must be evidence presented for that proposition. And then there must be a consideration of what the standards are for how great a threat a U.S. citizen represents.