I think the three most important aspects about WWII for a modern person are the following because they affect the way we (and the rest of the world) lives and still require active policy decisions (do we stay the course, increase, pull back, modify???):

1. Israel and the Zionist movement that led to the formation of the state. Although the movement began well before WWII the atrocities the Jewish people suffered under the Nazi regime were the final trigger the solidified the establishment of the state. Everyone has been dealing with the results.

2. Western meddling in Iran. The UK invaded Iran during WWII to stop Germany from obtaining oil. As opposed to the peaceful deal with the Saudi family (oppressive as they are), the Iranians had to deal with puppet governments until the revolution in the late 1970's which gets to the point we're at today where a historically advanced and progressive society that really should be part of the global economy has become the "enemy."

3. Beginning of big military/world police. Kosh made this point. I'd spin it a little more as an explanation for our military presence in all parts of the part. Additionally there is big money to be made with the large military policy and it's important for people to understand the implications and reasons behind the decisions.