Lend-lease was important, but in the critical 1942-43 period its impact was relatively small from what I've read. Certainly the vast majority of arms used on the eastern front were manufactured by the USSR.The USSR consistently out-produced Germany in all categories.
Your statement that "[The USA] took over the bulk of the military responsibilities of European allies" is WAY off the mark, that was always the USSR. While lend-lease may even have tipped the balance, it didn't add up to making the US contribution the majority effort in Europe. While I know this isn't strictly speaking your point in your response, check out the deaths per front in this http://commons.wikimedia.org/w... - it makes clear the overwhelming importance of the eastern front,
One bloodthirsty dictator beating another (slightly more) bloodthirsty dictator isn't quite as good a story as "America, World Police", but it's closer to the mark.
Overall in WW2 the USA did shoulder a huge responsibility of course, but a good portion of that went to the pacific.