The letter requests a formal report to Congress explaining the scope and legal justification for the drone program. "Authorizing the killing of American citizens and others has profound implications for our Constitution, the core values of our nation, our national security and future international practice," the authors write. "The executive branch's claim of authority to deprive citizens of life, and to do so without explaining the legal bases for doing so, sets a dangerous precedent and is a model of behavior that the United States would not want other nations to emulate."
The letter cites several problems with current drone policy, including its "unbounded geographic scope; unidentified high-level officials with authority to approve kill-lists; a vaguely defined definition of whether a capture is 'feasible,' [and] an overly broad definition of the phrase 'imminent threat,' which re-defines the word in a way that strays significantly from its traditional and legal meaning."
Full text of the letter can be found here: http://lee.house.gov/sites/lee.house.gov/files/President%20Obama%20Drones%20Letter%203.11.2013.pdf