well, as I see it, there are two questions here:
1. How would WW I go with an earlier Grentry

2. How Greek affairs would develop in the post-WW I era
most of the points already made by the posters here are well placed, but I'll contribute as well
1. An earlier Greek entry in the war means that there would be an earlier and more solid Balkan Front.
In the worst case, as it was mentioned, it develops more or less like IOTL, with the exception of a more "convenient" retreat for the Serbian army, and more troops engaged by both sides, which probably means less A-H troops for the Italian and the Eastern Fronts (?).
In the best case, a lot more British, French and even Russian troops deploy along the Serbs and the Greeks, they hold the A-H army and the Bulgarians, and they thrust into Thrace by land, probably capturing Constantinople, or at least forcing the Sultan into an armistice. After that I doubt the Bulgarians would carry on fighting, especially with the Russians in the neighbourhood. Also, more grain and supplies for Russia, with whatever butterflies this could bring (several TLs have coped with that).
2. An earlier Greek entry means no Dichasmos, ie the struggle between the liberals of PM Venizelos and the conservatives of King Constantine. Hence, a lot better odds for the Greeks in Asia Minor: even if we assume they lose it ultimately, they definately keep Eastern Thrace, and that means the Straits probably stay international.
Either the Greeks win or lose in Asia Minor, there would be a population exchange, as it had already happened with Bulgaria IOTL, but in a less severe fashion. And, it would definatelly include the Cypriot Turks.
In the interwar period the Greek politics would be more stable, since there would be no Dichasmos or a huge refugee flow. With the addition of the Cypriot Liberals, I can see that the monarchy will be more discreet in messing with politics and a solid Liberal sovereignity until the Great Depression at least. Nevertheless, a few post-1930 iritations would not undo a more stable and advanced Greece in the eve of the war.
Concerning WW II, Greece again would stay neutral in the beggining. But, Musolini in this case would think at least twice before attacking Greece. If Turkey maintains a position like IOTL, I can see Greece staying out of the war until the German retreat. If Turkey sides with Germany, though, there will be a mess: Italy attacks Greece in Epirus, Turkey attacks Greece in Thrace (or in Smyrna if Greece still owns it), Bulgarians and Germans attack in the North, Soviets attack Turkey through Caucasus and the British from the South (much like they did with Iran).
In any case, and if the Greek mainland falls, the political elite and the Crown would fly to Cyprus, and would maintain a more firm control of what happens in the occupied territories. So, after the war there is definately no Civil War and Greece gets whatever bits she desires: Northern Epirus (I doubt it would be ceded after WW I, due to the Italians), some more interests in the Straights, and the closure of (almost) the entire Aegean against Turkey. But the most important is that Greece would be probably less damaged and more stable politically, ie faster approaching with EEC and membership perharps in mid- or late 1960's.