Rowan Williams will soon leave his post as Archbishop of Canterbury bearing a slew of slings and arrows from attempting to lead the Anglican Communion through an especially raucous period of its history. His troubles parallel those incurred by all limited monarchs, up to and including Queen Elizabeth.
Since King John grit his teeth and acceded to the Magna Carta, royalty's power and prerogatives have been on the decline, balanced then overridden by the claims of constitutional government. George III managed to keep his head and the royal line in tact during the French Revolution mainly because the throne had ceded a good deal of its authority even then. The shining example of Queen Elizabeth as the epitome of a symbol without power is a singular achievement over style of substance.
The Primate of the Anglican cluster of churches around the world isn't quite as titular but almost. For all of the ceremony and accoutrements of office, the archbishop is as relatively hamstrung as any temporal ruler at the mercy, tender or not, of democratic constraints.
Every one of the constituent national churches of the Communion is independent and often onery. They may honor the wishes of the elegantly vested primate or ignore him. Constitutional government acts on a practical level without a meaningful parliamentary body to provide much consensus.
This is good or bad according to your understanding of ecclesiastical authority. It can be far less tidy that straight up-down hierarchy but, on the other hand, the conflicts which inevitably erupt in the process of churches constantly seeking to understand themselves are far less likely to be swept under the rug or allowed to fester. An excess of free-wheeling independence can be just as disastrous as too much coerciveness in an effort to establish the appearance of uniformity.
The next Archbishop of Canterbury will inherit some of the elegant trappings of the office and the willingness to exercise his duties without recourse to fiats or "my way or the highway" ultimatums.
Archbishop Williams has borne much criticism for allegedly refraining for using the power that he has, or shilly-shallying on several sexual matters of schismatic import. He also faced bold assaults to his office which were entirely beyond his making, including the Vatican's formal welcome to Anglican clerics who reject the ordination of women and gays, forcing him to make the best of an indignity worse than a case of bad manners.
But as he goes to a quieter place at Cambridge University, he takes with him a most valuable quality of leadership that no church constitution can give or take away -- the appeal of a deep thinker and kindly disposition to a larger world that sees so few of his learning and stature anywhere in Christianity.