I've got to say, this is an impressive necro. Alas, I cannot help but participate.
If Mussolini did remain neutral, WWII would be an entirely different animal. Continental Europe would likely be dominated by the Soviets (as a failed Anglo-American invasion, due to a lack of experience against the Afrika Korps, causes the Allies' entrance into Europe to be quite late). Then again, the Soviets would be tied down with extra German forces not in the Balkans/Africa in an earlier and potentially more devastating invasion. So Europe might be the same or dominated by the West or Soviets.
In contrast, I imagine that the Pacific theater would last a much shorter time than it did IOTL. No British forces needed in the Med. would free up a lot of the RN for combat in the Pacific and Indian Oceans, leaving the IJN fighting 2 fronts across two oceans (Indian/West Pacific and East Pacific). Hell, there may not be a Pacific theater at all (or at least as we know it); Pearl Harbor was inspired by the British attack on Taranto. Take this out, and the US either isn't involved in WWII (highly unlikely) or the IJN loses its only advantage (surprise/greater initial numbers) against the US, meaning that they're screwed.