Powell's problem is that he apparently thought he was the boss, and decided to say what he wanted, rather what he was instructed to say. Some of the worst diplomatic fiascos of the early part of Bush's first term were caused by Powell telling foreign leaders something quite different than he was instructed to. That changed with time as he learned his job, but his recent statement about Taiwan (which contravened existing and long-term US policy) was more of the same.
As for Rice, I think she's being groomed for far greater things, and with good reason.