Reading this thread, I have a couple things to add - and ask...
What's the fuss about the "mulberries"? An artificial harbor is of no use during an invasion (maybe because it needs to be built first ), only afterwards when huge amounts of supplies and additional units are landed. BUT - in the Pacific, the American already made dozens of landings without any artificial harbors installed afterwards. Right at the same time of the Normandy landings, the Americans landed in the Marianas - against opposition I might add - and didn't use artificial harbors afterwards either. Nor at Leyte. Nor at Iwo Jima. Nor at Okinawa. And American landings of course were unopposed (Solomons, New Guinea mostly) - as well as opposed, see Tarawa. In my opinion, the Mulberries were "nice to have", but not essential at all, and it's irrelevant whether WSC claims in his memoirs that he himself had a hand in inventing them. Just capture a harbor early, and you have your unloading facilities. Or do it the US Navy way in the Pacific, unload stuff onto smaller and smaller crafts which beach themselves or are able to sail in shallower water. Worked every time (okay, the drift at Iwo Jima wasn't all that helpful).
True, Churchill - and his General Staff - opposed earlier landings. Even D-Day was postponed in '44 for a couple months. Instead, Churchill favoured the strategy of attacking on the periphery (North Africa, Sicily, Italy). With hindsight, postponing D-Day was a good idea - see the half-arsed Dieppe "Raid" where a thousand of inexperienced Canadian troops were sacrificed for nothing (and nobody took the blame. Whose idea was this disaster anyway?). Would US and British/Commonwealth troops, with the experience, equipment, and tactics, of 1942 (North Africa) or '43 have succeeded in a cross-channel attack? Who really knows, but I'd say rather "No".
Could D-Day have failed? To my knowledge, not a single amphibious landing conducted by the Allies during the war, failed... Again, I'd rather say "No". Once the first waves are ashore, it was a - bitterly contested of course - sure thing. After all, the Allied had plenty of time to plan, plenty of time to build up forces, and plenty of experience from other theatres. They knew their job. They knew what they needed to carry out the plan. And they've got what they needed.
Last but not least, the one thing we western folks do not like to hear: at the time when the Western Allies finally landed in France, the landing was no longer needed to defeat the Nazis. Whether we like it or not, the land war in Europe was won by the Red Army (see dispositions of the Wehrmacht, the overwhelming majority of its assets were deployed in the East.
But what D-Day ultimately did - and this I of course say with hindsight - is to spare Western Europe the fate of... Eastern Europe. And thanks for that. Thanks to all the brave men, American, British, Canadian, Australian, French, Polish, Dutch, etc., be it Air Force, Army or Navy, who risked and gave their lives for this.
"A big butcher's bill is not necessarily evidence of good tactics"
- Wavell's reply to Churchill, after the latter complained about faint-heartedness, as he discovered that British casualties in the evacuation from Somaliland had been only 260 men.