How hard would it be for a modern soldier to fight in World War II with period tactics, weapons, etc

ACOGs and Aimpoints are practically bomb-proof, though. Short of smashing them with a hammer or melting them with a blowtorch, it's hard to not break them. And battery life on an Aimpoint is ridiculously good. You can leave a CompM4 (the current M68) on continuously for close to a decade on a single battery. And that's not even on night vision compatible brightness levels.

Not by any means arguing that they shouldn't teach troops to use open sights, though.

An M4 today is a whole different beast than ones from 20 years ago. SEAL Team Six recently ditched their piston 416s for a direct-impingement (same OS as the M4) carbine and Delta Force is trialing multiple carbines (at least one of which I know is also a DI gun) for the same reason.

 

I think it was mentioned above, it would be the specialists that would have the hardest time adapting.

Though the unique challenges that an infantryman would face would include navigation, vehicle mechanics, using radio equipment and RATEL procedures. They would also find it difficult to increase their library of hand signals.

I think the biggest challenge though, would be their reliance on logistics. It wasn't unusual for soldiers to go days if not weeks without some form of resupply. Where as since Korea and Vietnam most western nations have focused on a 48 hr supply line. That doesn't mean every man gets resupplied every 48 hours. It means that the time from which you request it to the time you receive it is no more than 48hrs. In WW1 and WW2 that simply wasn't an option.