The Italian fleet will sortie with the Austrians and join up with the Goeben with aim of stopping French army transports from North Africa. At the very least, this makes things more difficult and more irregular, even if there are no actual fleet engagements. The delays in the arrivals of some corps are going to have serious repurcussions for the French, especially if Italy also sends divisions to Alsace, and the Germans are able to make an advance in that quarter. Not to mention, that France has to properly defend High Savoy, even if the Italians are not making any serious attacks there. Their overstretch is increased, while their manning issues are made worse.
Italy may even attempt an invasion of Tunisia, either because the majority of French North African forces are being sent to Europe, or to try to prevent this and hold some of them in place.
Malta is going to be in a precarious position. The British Mediterranean Fleet is headed up by battlecruisers and if they get caught by an Italian force led by dreadnoughts, they are in for a very difficult time. I would expect they are withdrawn to Gibraltar, while London panics and eventually sends the Lord Nelsons as reinforcements.
While the Ottomans might never enter the war, Italian entry probably more than offsets this, as it focuses more forces on the essential Western front. Without any need to man a defensive border against the Italians, the Austrians probably manage a bit better and would certainly offset any late-year advantage the Russians have for not having to face off against the Ottomans.
Italy is going to intervene in Albania. This may create serious tensions with Austria, but since they are allies things will work out. It also puts Montenegro's position in doubt - they are related to the Italians by marriage, and it may be possible that after a show of defence Montenegro signs a peace without losses with the Central Powers and adopts benevolent neutrality towards them.
Serbia is more screwed by all of this and its possible that its army and royal family are not getting out, short of almost total annihilation.
There won't be a Dardanelles campaign - if the Ottomans are not in the war, there is no need. Even if they are persuaded to join, getting so many troops and old ships to the Aegean past a hostile Italian fleet, not to mention the Goeben and the Austrians, would just be too much of a risk.
The question would be why and what point would Romania and Bulgaria enter the war? While Greece might be the key, don't forget that Serbian Macedonia was in San Stefano Bulgaria and the Bulgarians had long agitated for unity with the Macedonians. Romania had a secret pact with the Central Powers that Ferdinand did not want to activate, but if Bulgaria enter the war to gain Macedonia, and the balance of power is tipping decidedly in the CP's favour (with or without the Ottoman Empire) then Romania could see its options shrink to the point where honouring the agreement is a better choice than trying to remain outside. With Serbia crushed, and Greece probably not in the war, the Central Powers could promise army corps to the Romanians to enter Bessarabia, even to threaten Odessa.
Best Regards
Grey Wolf