I think antisocrates has made a very interesting point about the Russia of 1914 not being Stalin's Russia and not having the same ability to move massive amounts of industry to the East.
As I believe the events of 1917 and 1918 show, a major German effort could take enough of the Balitc coast, White Russia and the Ukraine to cause Russia to crack and sue for peace. What if this effort had been made in 1914 and 1915?
A defense in the West could have caused the French to bleed themselves white while the major German offense was in the East, first to assist the Austrians and then to push the Russians out of Poland, the Balitc coast and into White Russia and Ukraine. Obvioulsy all of this could not be done at once but more could be done and more quickly than in OTL.
Making the major effort in the East can also mean no invasion of Belgium, no (or greatly delayed) British entry into the war and the benefits of no (or a later) blockade.