The same things GI's have talked about forever...the girl back home, what they're going to do when they get out, music, movies, how much they hated the Army.
I would get letters from my uncle with sketches he did of German tanks, airplanes, trains, and other things that were different than ours. He couldn't get postcards, so he did the same thing with drawings.
He had a macabre sense of humor regarding the horrors of war and it was reflected in his letters. After he came home, he said it was the only way to keep your sanity.
As in any war, soldiers developed a strong bond with their friends and he always talked about them, both in his letters from Germany and years later.