It was the most horrible tragedy to ever hit mankind, as far as I know.
Most of the technology developed in WWII would have been developed or was already being developed anyway. You like the roar of big engines, prop planes going 400+mph? Air racers would have done it. Jets? Already on the drawing board before the war. Atomic energy? Sure, it would have been developed, and maybe people would be more comfortable with it today if there had been no Hiroshima.
Another tragic aspect: The war started because of aggression by murderous authoritarian regimes against other nations, Hitler in E Europe and Japan in China and SE Asia. But in both cases, immediately after the war was stopped, E Europe and China fell under equally authoritarian and murderous Communist regimes. So no matter how noble the intentions of the Western Allies during the war, effectively all that was decided was a contest between equally odious regimes, the "winners" being Stalin or Mao, the losers being most of the populations who were suffering from Axis aggression at the beginning of the war. "When wolves and bears fight, no matter who wins, deer still lose." Europe West of Checkpoint Charlie of course, was better off, for that I am thankful. But still, as a crusade against authoritarian evil, WWII strikes me as a job that was only half-done.