Greenwald, who raised the alarm in the first place, explains why it's a betrayal:

To appoint someone as CIA Director or Director of National Intelligence who was one of George Tenet's closest aides when The Dark Side of the last eight years was conceived and implemented, and who, to this day, continues to defend and support policies such as "enhanced interrogation techniques" and rendition (to say nothing of telecom immunity and warrantless eavesdropping), is to cross multiple lines that no Obama supporter should sanction.  Truly turning a page on the grotesque abuses of the last eight years requires both symbolism (closing Guantanamo) and substantive policy changes (compelling adherence to the Army Field Manual, ensuring due process rights for all detainees, ending rendition, restoring safeguards on surveillance powers).  Appointing John Brennan to a position of high authority would be to affirm and embrace, not repudiate, the darkest aspects of the last eight years.