Greenwald's comments were in respect to the extra-judicial (as in there was no trial) killing of Anwar Awlaki, a US citizen and Al Qaeda Sympathizer who was living in Yemen at the time of his execution via a drone aircraft.
“If for whatever reason you trust Barack Obama, the benevolent constitutional scholar… do you trust that power when it is vested in Michele Bachmann or Newt Gingrich or Sarah Palin?”
I said long ago when the Congress refused to impeach Bush for his blatant violation of FISA and the 4th Amendment that the reasons this was being done was because the Democrats really didn't mind. They knew that once that power was given to the office of the presidency, that once they got their man or woman in the seat, they too could use that power. You will note that the Obama administration has often used the same arguments and legal reasoning as the Bush administration in regards to "anti-terrorism" policy. This is proof of this concern.
On non-presidential matters, I point to my objections to so called "hate crimes" legislation. Hate crimes legislation amounts to punishment for thinking. Laws are not supposed to be written, at least in the US, to punish people's thinking regardless to how offensive that thought may be. This includes speech that is not: 1) defamatory 2) pose an immediate threat to the general population.
In NJ we have already seen what happens when a "well meaning" law is put in the books (Bias Intimidation) that is used in a manner not meant or foreseen by it's supporters (some of whom are actually quite happy about it and wish to go even further).
I have seen a case in NJ where such laws were abused for the purpose of a politically connected professor who happened to be in the more "sympathetic" and "popular" protected class who was up against another person of a protected class. It is never pretty when the legal system can be used for bruised egos.
Partisans on either side of the general political spectrum need to be aware of these issues. If you put a certain law in the hands of someone or a group of people that you do not trust, would you still want it passed.