The two points I remember most vividly were the question of German vs. Allied fighting prowess, and the command decisions of Rommel, Rundstedt, and Hitler. I will tackle the first first. It is generally believed, according to Ambrose and to other historical accounts I have read, that the Germans were simply better fighters than everyone else in the war. Obviously, this statement has to be taken carefully: by the end of the war, the German armed forces were tattered remnants of their former selves, butchered by four years of war against the tremendous might of the U.S. and U.S.S.R. However, the argument goes, that over the course of the war, the Germans gave a better account of themselves than the troops of any of the other nations fighting in it.
Ambrose argues that this effect occurred because the Germans were primarily fighting on the defensive, not because their organization, tactics, training, or their innate capability as fighters. I am not sure I am convinced by this argument, both because of the outstanding offensive success of the Germans in 1940 and 1941, often against better equipment and larger numbers, and their performance in WWI, where the Germans nearly defeated an alliance of almost every other great power in the world against them: Russia was thrown into revolution and forced to peace terms, Britain and France were exhausted, and probably only the promise of the enormous resources of the U.S. caused the breakdown which ended the war.
However, the U.S. army had several factors which served to make it more effective: the Selective Service was actually selective, picking the most physically and mentally capable men in the correct age bracket; the U.S. probably had the best educated army in the war (more educated soldiers make better soldiers, and I've never heard a good explanation provided for this); the U.S. army was more technically competent than anyone, particularly servicing and repairing the all-important wheeled and traced vehicles; and the U.S. army had undergone an extensive training program, while most other troops (particularly after the start of the war) were more or less thrown into combat with a minimum of preparation.
Altogether, while Ambrose's point is well-taken, I'm not sure he's right. However, I'm not sure the prevailing opinion is right either. The U.S. army at the end of the war was probably the most powerful armed force in the world and vastly superior to the remaining Germans. At the beginning of the war, the Germans were almost certainly the best. If the two forces had met, other factors being equal, I'm not sure who would have won.
Ambrose also contends that, again contrary to the traditional view, Rommel's plan to defend the beaches would not have worked. He cites Rundstedt's analysis, which is that the Allies would not be weakest on the days of the landing, but immediately after, when they started to make their way off the beaches. The problem would be the enormous naval gunfire which could be focused on the beaches and immediately inland. He proposed to use the Panzer divisions later, to wait for the Allies to overextend themselves and then counterattack. Rommel disagreed, and wanted the Panzers for a first-day counterattack to destroy the Allied beachheads. Historically, Hitler split the difference, releasing some of the Panzers to Rommel but keeping the others in a reserve available for his orders.
The result, of course, was that neither strategy was carried out, particularly because most of the senior commanders were otherwise occupied on D-Day, including Hitler. In my opinion, it is questionable whether either strategy would have worked: Rundstedt was correct about the naval gunfire. Once the Allies had radios and spotter planes, anything in range of the coast would have been obliterated, in the same way that they later used land artillery (the U.S. fire-control system was one of their most effective tools in the war). However, the central problem with waiting was that any offensive undertaken had to be done under cloud cover: the total Allied control of the air ensured that divisions which were not dug-in would take such a pounding as to prevent them from assaulting anything succesfully. Rundstedt's strategy then depended critically on the weather. However, Hitler's split-the-difference decision can only be added to the list of truly disastrous mistakes he made during the course of the war.
This doesn't read very much like a review, does it? Well, think of it as a commentary. These pages are really mini-essays on my responses to what I read more than reviews. Take them as you will.