So, Germany doesn't have to get involved in Italy's failing adventures, but does have a relatively safe Southern flank, at least initially? There would be no need to devote all the troops to North Africa, with the Luftwaffe support that entailed. Nor any intervention in Greece, which led to the whole Balkan diversion before Barbarossa and threw the timing off.
This could mean that Barbarossa goes ahead on schedule with even more German forces available (what role would Rommel have for example) and Germany could actually achive its aims there
IIRC from looking at the issue years ago Barbarossa could start about 7-10 days earlier without the Balkans campaign thanks to the weather. That's somewhat helpful, not not necessarily decisive. However the bigger change is just how many more forces would be ready to go instead of worn down from being in the campaign in the Balkans or not available at all due to occupation duties, damage, and just ability to get out of Greece and back to Germany in time. Like 2nd and 5th Panzer divisions...though they were helpful in October when they were the only full strength panzer divisions to show up fresh and ready to go for Typhoon (not that that changed the outcome) and were IIRC about the strength of the average worn down Panzer corps each.
Here is a US Army study of the campaign and it's effects:
That said the Yugoslav invasion may well happen here ITTL because a lot of the same causes were there, but that was wrapped up by May and much easier to extract from, plus less costly than the entire Greek campaign (initially). Without Italy though it would require a fair bit of extra forces from the Germans, but it is hard seeing Italy remain 100% neutral on the dismemberment of Yugoslavia unless Mussolini is dead and even then it may well still happen, though having Britain declare war on Italy for that would probably be rather tough.
There is also the issue of Germany not controlling Greek resources, which were not insubstantial or valuable. They had chromium deposits that provided about 25% of German needs during the war:
en.wikipedia.org
Not only that, but food and money were big helps exploited from Greece, the former being so exploited that Britain was forced to lift the blockade of Axis Europe to allow food aid in for the people of Greece to help blunt the famine that was starting.
Certainly Germany could buy from Greece, but given the Allied economic warfare program that may well not be a viable option to get what was gotten IOTL. Though without the Italian invasion Greece might well remain pro-German. Of course that would only really matter in the long war, not the course of Barbarossa and it's direct aftermath.
Even with Yugoslavia still being invaded, this time without Italy, but not Greece Germany would gain vs OTL quite a bit especially without a Mediterranean campaign. Depending on Italian leanings they could well help Germany by selling/bartering with them for a lot of stuff, while Germany is absolved of effectively gifting them resources to run their war effort. Reading a published diary by a White Russian ex-noble living in exile in Berlin Italian restaurants were extremely popular and potentially vital to German diets in 1939-40 before Italian entry into the war because they were not subjected to rationing at all as they sourced their food from Italy, which was not rationing at the time. Given how much rationing hit Germany hard before the victory in France gave a temporary respite when they were plundered, this would be a pretty big help to the average German longer into the war. Not only that, but Italian workers would find plenty of employment in Germany the longer the war goes on and were quite a substantial part of the workforce in Germany throughout the war IOTL; here they may well even be a bigger one.
In terms of forces something like 1/3rd of the Luftwaffe was committed to the Mediterranean in 1941, though this fluctuated during Barbarossa, which would be a big help in the East. The biggest help is the extra several hundred Ju52s not lost during Crete/Greece and not retained to help Rommel in the desert. Not just that, but the forces saved from the Desert Campaign are available in the East, which IIRC ended up being something like 10% of German trucks in 1941, though to be fair a substantial chunk of their logistics were provided by purchases of French trucks in Tunisia and provided by the Italians. Perhaps Italy starts selling Germany trucks ITTL? A lot depends on where the extra forces are applied ITTL; per Halder's diary and other sources it seems that only AG-North has the logistics, thanks to Balkan shipping as well as the road and the rail net in the Baltic states to handle the extra forces and trucks, which when coupled with the extra Ju52s could well generate strategic benefits over OTL that might be enough to push the Soviets over the edge. Hard to say for sure, so many variables.
However Britain also gains a lot too. Italy being neutral saves them pretty huge shipping and manpower resources, though to be fair they lose a lot of the experience they gained IOTL in the desert and Greece. This likely means that they might well try and invasion somewhere in Europe in 1941 or '42 because of lack of anything else to do. It will probably go about as well as Dieppe if not worse, but provide important lessons for later and may well be repeated in 1942 or sooner. Hard to say on balance what that would mean for them, though the savings in forces probably really helps in Asia and the Pacific, while helping the economy by maintaining easier contact with the Empire. They could also get themselves in trouble when and antsy Churchill gets them into 'adventures' during the years before the US is ready to help. Though IMHO assuming the US comes in at the same time a 1942 invasion of France is basically a lock...with all that entails.