If by some act of sheer bravery, tactically genius and total luck, the first german wave was repelled then Greece would of shocked the world. Politically the axis will be humanilated and Hitler, in rage, will no doubt order a mass bombing as he readies more troops for a second invasion. This will severely weaken the Eastern front and delay it even further.

Also, the British may pour in reinforcements and wil strength the Greek front, Italy will retreat further and will even be endanger of being pushed back to the sea.

Although Greece will no doubt fail as the German hammer blow eventually falls, this will delay the Russian ivnasion so much it may be post-poned till after winter, this will have alot of changes, the Russians will be better prepared, but on that same note the Germans will have almost a whole year to take Moscow, and by then the high command could of also changed they plans for the better, as Manstein did at the invasion of France.

On another note, the Soviets could of been alot more prepared, and even push the Germans back very quickly, ending the war rapidly.

Or the allies will take stock of this and use Greece as a major landing point, strengthing its defenses and even making probing attacks in Yugoslavia as Hitler turns his eyes east, but then again this would of weakened the Africa and home front.

The list goes on and on...