I've seen a lot of best and worst lists as I've browsed around on the web. I'm wondering what were the biggest opportunities that were passed over? What prototypes or modifications should have been followed through on?

I've seen the M27 dismissed as simply a faster M4, but it was more than that. Faster, yes, but it also had better traction, mobility, efficiency, armament and a lower profile. It had slightly improved armor as well. 60% parts commonality with the M4A3 as well.

The P-38K variant of the Lightning. Adding a larger propeller and minor modifications to the nose cone and cowling gave a 30 mph speed increase and a 15% increase in range. Lockheed begged and pleaded to make the modifications. They would have had to stop production for two weeks (their estimate) to make the changes. They were told no.

There must be lots of other choices and reasons.

the british command turning their nose up at the jet engine

How about the U.S. and it's ressistance to explore the probability of the aircraft carrier at first. Heck, even tanks got the brush off at first. Seems people get comfortable with an idea and that's it, oops, we just got our butt kicked, what do we do now.What was that saying about hindsight?

Hitler's insistence on developing the Me-262 as a bomber rather than an interceptor?

Hitler/Luftwaffe ignoring the need for a long range heavy bomber until it was too late?

C.