I'd say the biggest change is that we haven't fought a war on the scale of WWII since then, as most of our engagements have been with inferior forces (like the Gulf War) or were unconventional/counterinsurgencies. Of course, drones and airstrikes have changed things, and allow us to do a lot more with less ground troops--but we've always had air superiority. Russia has a larger and better equipped air force than us, while we don't have enough pilots and half our aircraft are sitting in hangars being scavenged for parts to keep the rest flying. All of NATO combined only has about half the forces that Russia has, and we would probably be stretched thin at Iraq war surge levels just to fight North Korea and Iran at the same time. I doubt we could raise troop pay/benefits sufficiently to recruit enough soldiers through volunteers alone.

Hopefully I'm wrong, and hopefully we don't ever have to fight another large scale war. I'd like to keep the draft a memory. But if we do, I expect we're going to need huge amounts of personnel and materiel.