The general consensus is that the greatest obstacle to a successful invasion of D-Day was the problem posed by the English Channel.
An invasion across the English channel had only been done successfully once before in history, in 1066 when William the Conqueror led the Normans across to defeat Harold at the Battle of Hastings.
The English Channel provides a number of logistical and tactical problems in that it is difficult to cross, in some weather, but also that the opposing army is given time to prepare.