Eliezer makes some very compelling logical arguments on the case for approaching AI with extreme caution. It stands out to me as an odd signal that that he chose to refrain from signing the letter because it asks for too little. If one were truly convinced that AI would end humanity, signing the letter to put the research on hold seems to be one tangible step that could be made in the direction of saving the human race. As someone who is very concerned about safety, to me this detail stands out and appears as grandstanding.