They inflicted heavy losses in the West in 1914, much higher than they received themselves, overrunning most of France's raw material base and much of her industry outside of Paris, while preserving a higher percentage of her pre-war army than any nation in the war. In the East the Russians had a very good army, better equipped than the Austrians by far and in fact having a better artillery part per division and corps than anyone but the Germans. The Russians really weren't that disorganized either; Tannenberg was a mess, but most of the problems the Russians had was strategic and morale-wise, rather than tactical or even necessarily operational.
The Germans outperformed in 1914 due to having a better army and preserving so much of it until 1916 and the Somme. Then Germany was badly outnumbered and was keeping its collapsing ally in the war from 1914 on. The issues with getting the US into the war stemmed from faulty intelligence about the political situation in the US and would have been rational (other than Zimmermann admitted his telegram was real) had their read of US politics been correct. Really if you want to know why Germany did so well after 1914, as in 1914 they were on the offensive in the west and did better than anyone else during the entire course of the war in that year, it wasn't just that they were sitting on the defensive; it was that they got their industry organized first to produce sufficient artillery shells, had siege warfare equipment pre-war that really worked well in trench warfare (howitzers/mortars), seized an awesome defensive line on the Aisne and withdrew to it, adapted their defensive doctrine to the realities of the situation as the war progressed, were willing to give up ground to disjoint Entente offensives (1917), and just had a low quality series of enemies in the West until 1917. If anything the question should be why did the Entente do so poorly until they developed a numerical and material advantage in 1917?