Daniel Drezner has a very good post on the diplomatic game (scroll down to "AND SO, THE END IS NEAR...") and Colin Powell's role in it. Drezner linked to this article from the New York Times and this article from the Washington Post on the subject.
The bottom line was this: Powell held that policy making in Washington D.C. was more important than selling American policy over seas.
The traditional role of the Secretary of State is that of a traveling salesman for American policy. It is primarily an operational role and not a policy making role. This has been true for decades. The reason is that the State Department bureaucracy views diplomacy as its primary role. It won't do anything else, even when ordered to do so. This is where the old saw about the State Department needing an 'American Desk' comes from. So managing State has been a pointless task that Secretaries ignore to jet set.
Powell will not travel and do his job as Secretary of State because he is too busy with the policy making debates at home. This was his failing as Chairman of the Joint Chiefs in Bosnia, usurping elected and other appointed policy maker's decision making authority. It will be his epitaph as Secretary of State.
I always thought Tim Burton had the most scathing review of Powell. I could be wrong, but when I saw "Mars Attacks" I thought General Casey (the officer sent to greet the Martians) was a direct jibe at Powell. The memorable line was something along the line of: "Didn't I always tell you honey, if I just stayed in place and kept my mouth shut, good things were bound to happen."