This is almost (but not quite) ASB. But its fun to speculate. If for some reason the US was predisposed to the Central Powers and immediately entered the Great War on their side in 1914, the entire thing would be come an unpredictable clusterf**k. First, there would be the real possibililty of war (a la Turtledove) along the US/Canadian Border. Britain would have developed a more militarized Canada or there woud be British troops stationed there. Likewise the US would have to concentrate forces in America. This might limit US military nvolvement in Europe to assist the Central Powers.
The strategic situation regarding naval war might the most interesting difference, however. With a powerful US Navy in the Atlantic, coupled with the High Seas Fleet threatening Britain in the North Sea, the Central Powers could maintain a much more thorough and agressive war on British commerce without resorting to unrestricted submarine warfare. On the other hand British naval forces based in Canada could stage raids on US ports coastal shipping and the Carribean. With two major bluewater fleets in action against each other in the wide Atlantic and scouting of great importance, it is possible earlier and more extensive introduction of naval aviation (either carrier based or perhaps via airship) might occur.
Presumably, the US would also have a much more sizeable presence in the Pacific than Germany did OTL, and one might see interesting actions between the Japanese (who would be about seizing US colonies and holdings in the western Pacific as well as German ones). They might even go after Hawaii, which was not yet a major US naval base. Most likely, with a (presuambly even larger than OTL) Royal Navy threatening the US in the Atlantic, the US might let the Japanese have their way at first.