In James Comey's testimony before the Senate Judiciary Committee he said that Attorney General John Ashcroft was prepared to resign if President Bush authorized an illegal program against Justice Department recommendations. See Talking Points Memo for the video.
Ashcroft's courage and integrity contrasts with the "go along, get along" behavior by Secretary of State Colin Powell. Powell probably deluded himself into thinking he was retaining influence by staying in the Bush administration, but it seems unlikely he ultimately influenced policy.
One could argue the timing made a difference. Powell lied to the United Nations February 6, 2003. Ashcroft stood up to Andrew Card and Alberto Gonzales on March 10, 2004. While Powell's dishonorable behavior was closer to the 9/11 attacks, it was a full year and a half later.
Anyway, it seems noteworthy that there were people who stood up to Bush in the administration. There were people who threatened to resign. Colin Powell wasn't one of them. And ultimately he had little to no influence because he didn't threaten to resign.
Colin Powell may have been popular and articulate, but when faced with the choice of what to do, he took the path of least resistance.