The necessity of dealing with the Greek resistance and subsequent British intervention (the necessity being to protect the all-important Ploesti oilfields), took vital time away from the Axis forces - slowing down the onset of Barbarossa and also mildly denuding the forces available for its success.
The failure of the invasion of Russia, which indeed could have been a success, was the deathknell for Nazism.
Another massive factor in its failure was - Pearl Harbor. If the Japs had attacked Russia, as their army faction were minded to do, then Kutuzov could never have brought his Siberian troops back in time to defend Moscow. The USSR would have collapsed and any eventual D-Day would have faced vastly more potent tank forces.
Instead the Japanese Navy launched the militarily brilliant, but strategically toxic attack on Pearl Harbor. Kutuzov was free to move his forces, and of course the sleeping giant awoke.
If Hitler had our hindsight, he would have cursed Yamamoto every bit as much as he cursed Mussolini.
Yup. We lucked out.
Had the Nazis conquered Russia, an Anglo-American invasion to liberate Europe using conventional arms would have been quite impossible.
WWII would have still ended with an Allied victory, but probably only after Berling and a few other German cities became radioactive.
There is another aspect of the invasion of Greece that I alluded to the other day though that I think is very significant.
As we have seen the last few days, Hitler has been trying to secure the support of Vichy France as well as Spain. Franco was really on the fence at this point, but the Italian invasion of Greece helped him make a decision to stay neutral. After all, how could he rely on the promises of Hitler when he couldn't even keep his allies from running off and starting new wars with other neutral countries.
Imagine if Spain had signed on with the Axis. There is a good possibility that Gibraltar would have been attacked by land and the entry into the Mediterranean would have been effectively sealed from the British. This would have completely changed the game in that theater which may have lead to the collapse of the area giving Italy it's desired private lake. It also would have taken the command post away from Eisenhower for the TORCH landings two years later. He would have commanded initial operations from London.