What were the flaws in the plans and execution of the D-Day invasion?

Understand, first of all, that this was a massive
undertaking, the largest seaborne invasion in the history of man.  There were bound to
be problems and flaws, and as has been famously said, no battle plan survives the first
day of war.  Operation Overlord was no different.

There
were some glaring mistakes in both the plan and the landings.  Allied intelligence
greatly overestimated the success of pre-landing bombings and offshore bombardment on
German defenses.  Other than disrupting rail traffic and inhibiting reinforcements, it
had virtually no effect at all, and the landings met with fierce resistance and high
casualties.  This was a lesson we should have learned from our Pacific experience by
this time.

Allied intelligence also underestimated the
amount of anti-aircraft fire that would greet the airborne soldiers landing the night
before D-Day.  This forced the planes containing the paratroops to fly lower and faster
than was ideal for a jump.  This led to a slew of missed drop zones that scattered the
101st and 82nd Airborne divisions all over Normandy.  They carried out their missions
regardless, but with unnecessarily high casualties.

The
plan also relied on simple blunt force: land as many troops as possible in as short a
period as possible.  Smaller, specialized commando units that could perhaps more easily
have penetrated the Atlantic Wall could then have secured beachhead access for a larger
landing later (and there were some of these raids, just not enough in my opinion to
avoid the oncoming slaughter).  Instead, thousands of British, American and Canadian
forces died that day.