The Ottomans and the Russians had a long term thing going on. So the Russians recognise the Greek claim to the islands to piss off the Ottomans (the shiny new British built modern battle ships about to upset the naval balance of power in the Black Sea could also act as a catalyst towards this). Of course the Balkan League nations are delighted by the Russian position. This in turn concerns the Austrians who see the Balkans as their sphere of influence.
So far this ratchets up tension but doesn't provide a
casus belli, unless A-H or the Ottomans issues an ultimatum to Russia to withdraw the recognition?
Naturally the Central Powers who had been quietly improving their relations with the Turks are horrified by this Russian interference as the islands are clearly Ottoman. Further they proclaim that the Tsar is sabre rattling and they are unimpressed and unafraid. The breakdown in relations forces the Russians to go gear up their war machine just in case. However as no system of part mobilisation has ever been figured out for the Russian military the chaos that follows convinces the Central powers that the Russians are preparing for an war and once they all start mobilising in turn war general becomes inevitable.
Well there was a partial mobilisation plan for the Russians, in fact two. One for a war against just Germany and one for a war against just Austria-Hungary. The problem is, if the Russians activated the purely A-H plan and then the Germans mobilised, they wouldn't be able to react until the A-H related mobilisations were complete. Mainly due to the lack of rolling stock and poor co-ordination of railway lines, their "Great Plan" of military improvements were partially targeted at rectifying this. The general staff knew this and manipulated the Tsar into proclaiming general mobilisation to cover the possibility of Germany mobilising. This in turn made Germany mobilise as Russian units against their border were mobilised. It's the problem with all the nation's mobilisation plans - they were often viewed by the mobilising nation as precautionary, even defensive, in nature and as provocative, even aggressive, by the nation being mobilised against.
The main war plans had been drawn far in advance so the difference in flash point particularly for the French, Germans and Russians doesn't change their opening moves so the main battles of 1914 go as OTL.
So, War follows mobilisation, even though all that happens is proclamations have been made about the Aegean Islands? Germany plunges into Belgium, just because the Tsar says he agrees that the Greeks should have Karpathos and Kassos?
The Balkans may go a little differently. As Greece and the Ottomans are more likely to be in it form the beginning. This means there would be an active front fought in Thrace between the two. If the other Balkan League nations get involved then they could/would face the Austrians as well. The Serbs managed to put up a decent fight against the Austrians IOTL. ITT aided by the Greeks they would at the least have somewhere to retreat to. I would expect western forces to be deployed into the Balkans (as was done later in WW1 OTL). Another variable is how this plays out in Bulgaria and Roumania. Neither of these nations had any traditional love for the Turks. If the Bulgarians stay neutral or worse join the Entente this would be a blow to the Central Powers as OTL the Bulgarians where first rate soldiers and it closes any assistance they could give to the Ottomans.
I should hope the Greeks are "in it from the beginning", or else what has started the War? By 1914 the Bulgarians had less love for the Greeks and Serbians than the Turks, I would imagine having recent grievances against both for territory "lost" in the second Balkan war.
By 1915 the Balkan front starts to create a divergence from OTL. Do the British and French reinforce the Balkans instead of fighting at Gallipoli? Or Both? If Gallipoli goes ahead does the fighting to the north west tie down the Ottoman reserves so the line crumbles? Could this knock the Ottomans out of the war in 1915 as planned? Does this active front encourage the indirect rout supporters in the British cabinet to reinforce the Balkans at the expense of France and is this a war shortening move or not?
My gut says it would be to the allied advantage as the Austro-Hungarian Empire is far weaker and plagued by internal ethnic factions than Germany and if the Entente focused on knocking them out of the war before the Russians tire and descend into revolution then I just don't see the Germans fighting alone from say 1917 onward's.
If, and it's a big if IMHO, the war starts as you've said, an earlier and bigger Balkan Theatre is highly likely. But, if War follows largely as OTL as you've said elsewhere, do the WAllies have the troops to make a difference? How is the performance of Austria-Hungary any different in this war as to OTL?