How important was D-Day to the outcome of WWII?

The answer would depend on what one means by the "outcome." It is vital to the division of Europe into NATO and the Soviet Union as the Allies progress on the Western Front was important in determining that division. However, if the outcome simply means that Germany and the Axis losing the war, DDay is not necessarily that important.

The economic and strategic trends that led to the defeat of the Axis in both Europe and the Pacific were well on their way by the time the Allies landed in Normandy. The growth of industry in the United States that would produce the massive material advantage in the Pacific would combine with the overwhelming manpower and eventual technical parity on the Eastern front brought to bear by the Russians.

So D-Day served to hasten the end of the war in Europe but had it happened at a different time or not necessarily happened at all, Germany was still on its way to defeat.