I'm really interested in hearing more about the power vacuum following Cnut's death. Edward the Confessor had a fairly lengthy reign in England in the interim, but never seems to have consolidated any sort of lasting power. Why was William able to essentially step in and take charge? Did he get lucky? Is it a case of right-place-right-time? And why did it last - how did the Normans hold on after he died?
The problem with Edward's reign was that he left no heir to take the throne after his death, leaving it open for other claimants. Harold Godwinson was elected king by the English nobles, William claimed that he had been promised the throne by Edward while he was in Normandy during his own period of exile (and that Harold and sworn to abide by this [both accounts are disputed]). Harald Hardraade also put forth his claim.
William however did not simply step in and take charge. He had to fight the battle of Hastings and engage in a lengthy campaign to suppress Saxon rebels. William had significant difficulties firmly establishing his rule over England as he faced rebellions from Anglo-Saxon lords and incursions from the Scots as well as displaced members of the Anglo-Saxon aristocracy. I can go on about this, but it seems to be outside the scope of your question.
However, the overriding reason why William was able to establish himself so easily was that the brothers, and the most obvious heirs apparent, of Harold Godwinson all died in the battle of Hastings or were rendered powerless soon after it.
Tostig, a former Earl and Harold's brother, was killed in the battle of Stamford Bridge, Harold, Leofwine, and Gryth were all killed in the battle of Hastings. Wulfnoth was made a prisoner shortly after the battle and seems to have remained in captivity til his own death. This made all of them obviously unavailable to resist the Normans. This left the royal dynasty essentially neutered.
Harold's sons fled and made an attempt to reclaim the kingdom by allying with a welsh noble, but they failed and soon faded into obscurity.
Edward's closest male relative at the time of Hastings was too young to be considered a serious contender for the throne, despite an attempt to crown him before William arrived in London, and he spent a great deal of his later life fighting for and against the Normans.
Sources:
Mason, Emma. House of Godwine: The History of Dynasty
Hale, Betty. "History of Prince Edgar & his Claim to the English Throne"