Let's assume, for convenience's sake, it starts as something between Austria and Serbia and spirals into a European-wide conflict, with, at the bare minimum, Germany/AH/Turkey on one side and Russia/France on the other. The biggest shifts, IMO:
- Russia has had an extra decade to industrialize, and is probably the most militarily powerful country in continental Europe. This alone could shift the balance of power firmly in favor of the Entente.
- Italy and Britain may or may not have left their respective alliances.
- We're keeping Austria-Hungary alive for the purposes of this discussion, but Franz Josef is probably dead and Franz Ferdinand is attempting to implement reforms.
- The Ottoman Empire (which we're also assuming has stayed whole) has also had an extra decade to industrialize with help from Germany.