Why was the Schlieffen Plan significant to World War I?

The real significance of the Schlieffen Plan is that it was short-sighted, and actually ensured that the war would turn into the bloodbath of a general European war. The plan, as well stated above, was for a massive preponderance of German forces (including reserves mixed with active units) to swing through Belgium and across the north of France, enveloping the French Army outside Paris and ending decisively within six weeks.  Initiated by Count Alfred von Schlieffen long before the war and finalized in 1905, the plan called for 34 divisions to form the right flank, a division being a force of approximately 15,000 men usually commanded by a major general.  Only small forces (eight corps, 320,000 troops) would mask the defenses of Germany in the south, just enough for a defensive battle, and leaving mostly token forces in East Prussia to hold off the Russians until victory was won in the west.  The problem was Belgium and Belgian nuetrality.

A neutral Belgium was the creation of Great Britain, and the Belgian Army although small did have the capability to throw off the finely-tuned timetable of the German General Staff.  Belgium under King Leopold probably would not have fought the Germans, but his successor Albert was quite different.  His forces did hold up the Germans, although very briefly, and the violation of Belgian neutrality of course brought Great Britain into the war.  The Germans expected the war to be over so swiftly that this would not matter, but a series of incidents intervened.  The Germans worried about Prussia and bled forces off the west; they were so successful at throwing back French attacks in the south that they pushed forward and in turn were rebuffed, causing more reinforcements to be withdrawn from the right wing; the pace of the German advance on the right was so fast it exhausted the troops; and most of all, the British Army did not act in the expected manner.  All together, these things caused the German forces to falter at the Marne, and when the British (thought to be beaten and withdrawn from battle) appeared in the gap between the two right German armies, the offensive collapsed.

With this it became impossible for the Germans to end the war quickly, and with the stalemate of trench warfare subsequent to the "Race to the Sea" the war bogged down.  The sea power of Britain then became the dominant factor, and the slow strangulation of German food and materials from overseas eventually ground down both the military and the home front.