Anglosaxon England was managed relatively well, and had a relatively strong army. So why did it fall to the Norman Bastard, and why was the end result not simply a change of dynasty, but a much larger political change? First, we must apportion prime significance to luck and conjuncture. One could well argue that had Harold Godwinson not had to deal with Harold Hardrada's landing in the north, his forces would have been better filled out, more rested, and more completely arrayed to deal with the Norman invasion. Anglosaxon warriors were no less accomplished at their art than were their French opponents. More concretely, some factors suggest themselves: a) for the generation prior to 1066, a series of earls had started to come between the King and several shires, perhaps decreasing the royal domains or revenues. Definitely, the rising strife weakened the body politic's cohesion and unity. b) William was able to nullify his numerical inferiority against Harold by hiring Norman and other French knight-adventurers. Primogeniture in passing on feudal possessions in France meant that large numbers of second and third sons were trained as warriors, and in need of new prospects. Thus, William was able to gather a large host. c) Godwinson's army was certainly enervated after engaging Hardrada, and the English king did not wait for reinforcements. d) It may be that the heavily- armored, mounted French knights had tactical superiority over the Anglosaxon bowmen and infantry. This is not certain though, as the Saxons held out for quite a while. Perhaps, though, attrition in this kind of combat would deplete Anglosaxon forces more quickly than would be the case for French knights. Such a pattern would occur during the Crusades.
Before dealing with systemic changes in Norman England, we should also look at the 1066-1070 revolts. Why did they not succeed? 1) There was no candidate for Anglosaxon king that could unite all the rebels. Harold and his family had been killed, and the Witan was no longer active. 2) In the same way, the organized Anglosaxon army had been wiped out. 3) The revolts were mostly in the North, in and beyond Yorkshire. Here the population was thin and mostly non-warrior. Also, this region was far from the real centers of English power. 4) Swen Estrithson's landing convinced William that the threat to his rule was quite real. What might have otherwise been a war of attrition turned into a major Norman onslaught, a key focus of which was rural destruction for its own sake-- the denial of a material basis for northern residents.
This last point hints at the nature of the change from Anglosaxon to Norman. In the north, there was simply no longer an Anglosaxon basis. This also occurred on the elite level. By apportioning lands to Normans, and by 'reforming' the English church, William decapitated the tops of Anglosaxon society, replacing it with Normans. Northumberland became a march on the Scottish border given as fief to a great Norman lord, just as nearby Durham was given as fief to the area's newly appointed bishop. Thus, the change consisted of the creation of a system much more like the European feudal model, yet with divergences similar to the English precedent that gave Norman kings much more control over their realms. On the one hand all lands were now held as fiefs, and secular jurisdiction was separated from ecclesiastical, as was the European case. Further, the Norman curia, or assembly of feudal vassals, replaced the Witan. Also, Anglosaxon landowners who were allowed to remain in place now faced new feudal burdens, just as small farmers who had previously been nearly free were now subjected to a Norman lord and went through a process of semi- enserfment. On the other hand, vassals of all levels were made to swear that their feudal loyalties were not at al to come before loyalty to the king. Further, William had come to possess Anglosaxon royal domains, and was the wealthiest man in the realm. He could thus demand regular feudal knight service for any campaign. Also, he maintained or strengthened Anglosaxon practices. Popular courts continued to administer justice, and the offices of sheriff remained, now occupied by Normans. That the king appointed these sheriffs allowed him to control local administration much more closely than could feudal leaders in Europe. As well, the Anglosaxon chancery was maintained, as was the right to levy occasional taxes, and the prohibition of barons' private wars. The best testimony to this feudal centralism was the Domesday Book of 1086. Royal commissioners held sessions in each county and then calculated al the revenue resources of a district, as well as its feudal relations and military potential. This provided the sovereign with a wealth of information about how to use best his realms.
William Rufus and Henry extended these processes. Rufus kept barons in control by financial extraction. He also began replacing barons in sheriff positions with men of much lower social rank. Henry completed this process. As well, royal justices sent from the center began to adjudicate more and more cases that used to be dealt with on the local level. Thus, both sheriffs and royal justices were chosen from people without local social status, and they would owe everything to the crown. Henry also centralized financial administration. Before him, financial reserves were simply kept in a strong castle, while current funds were stored in the king's own bedchamber. Henry sent three of his chamberlains to the Winchester castle, along with the current funds. One official became the treasurer, while his assistants were the chamberlains of the exchequer, maintaining better control of central finances. As regards royal revenues in the shires, a body of Barons of the Exchequer mad twice yearly, at which sheriffs accounted for expenditures and their need for reimbursement by the crown. Thus, Norman England was indeed feudal, but by combining Anglosaxon practices with innovations to remain masters in their own realm, English kings were much more powerful than their French, or perhaps even German counterparts. Still, all this was accompanied by an overall depression in the socio-economic conditions of the average Englishman in comparison to pre-Norman times.