At the time it was not readily apparent, just as
@Scott Washburn notes, but at the time, Russia was at the greatest relative military disadvantage since 1600 or thereabouts. Russia was still absurdly
big, and this was historically its great advantage (strategic depth + vast numbers), but its military wasn't ready for a war against highly modernised Germany. OTL demonstrates very,
very clearly how big the discrepancy was: Germany managed to knock Russia out of the war even though it was dedicating its main effort elsewhere, and while also fighting several other great powers.
An "east first" strategy, if handled competently, was a brilliant idea. Von der Goltz knew what was up, and they should've listened to him. (Falkenhayn came around at the last minute, but by then it was too late, and Germany was committed to its idiotic 'France first' plan.) To be fair, Germany didn't
know (initially) how great the Russian disadvantage was, and prior experience against Russia had led to sensible caution. The fact is that 1914 provided an almost unique chance to actually beat Russia swiftly and effectively, and Germany predictably squandered said chance. If it had happened otherwise, things could still have gone wrong. Particularly, a very serious attempt would have to be made to spread disinformation, and to make both France and Russia believe -- for as long as possible -- that Germany intended to go "France first". If that can be managed, results in the actual opening stages of the war can be maximised.
What would this have meant? It would have meant a strong defensive position for the Germans in the West, forcing France to either throw itself into the meat-grinder of Elsaß-Lothringen in the (rather uncertain) hope that Britain could be convinced to join on their side, or to attempt to widen the front by marching through Belgium (which would near-certainly make British entry on the French side impossible). In the East, the initial German offensives would be costly for them (they'd be attacking the Russians, instead of the other way around), but would be overwhelming in their strategic success.
Basically: CP victory becomes inevitable by mid- to late 1916. Britain stays out (even if they still
want to enter on the French side, the war will be over before a clear majority for this can be constructed). The USA never gets involved. The peace is relatively 'white', because the war is shorter. Lenin is never sent to Russia, but the Tsar is still rather discredited. Constitutional monarchy ensues.