I'm intrigued by this thread but a little disappointed by some of the 'off the cuff' answers which seem to have based more on individuals patriotism than through thinking through of the original premise.
Therefore, Germany didn't win WWII.
The U.K had been steadily releasing it's empirical hold before then (as pointed out by Zen_Tom) but has never really been a super-power to begin within.
After the war the U.K and Europe would rebuild and form a European Union (just like real life!), whether the U.S is there or not. This union of countries working in cooperation would (and will) benefit thousands if not millions of people. So I don't immediately see why without the U.S (in reality or otherwise) the world would automatically qualify as being 'much worse off'.
Now, leaving politics and patriotism to one side.
If the U.S didn't exist after the war then Rap, Grunge, Hip-Hop, House, Elvis, The Beach Boys, Rock & Roll and Motown music would all be words without meaning for us now. Although The Beatles might still be together if they could have taken their spark of creative muse from alternative sources...
Most importantly and feel free to pick me up on this, but there would be no doughnuts and really that would make the world a terribly sad place in which to live...