Fantastic Four #4
This is the return of the Sub-Mariner from the Golden Age of Timely Comics. He returns after being discovered by the Human Torch in the Bowery. There is also another atomic bomb explosion in this issue, showing more repetitive plotting.
Another motif in the early Fantastic Four comics is Susan Storm as the damsel in distress. She usually contributes very little to the fight scenes (well, I mean, all she can do is turn invisible, so I guess what do you expect?) and frequently ends up needing to be saved by the three (male) members of the Fantastic Four. It's pre-second wave feminist sexism at its finest. I'll be curious to see the way comics changed their views on women (as well as blacks and other visible minorities) starting with the late '60s issues.
Also, for all the historical talk of the great character development of the Fantastic Four in these early issues, I don't see it. Granted, it's probably better characterization relative to what other comics were doing at the time, but comics clearly had yet to mature as an artform. I suppose it was one step in the right direction, although Namor's reaction to Susan Storm after he captures her and she becomes visible is hilarious in its sexism. Check it out for yourself (she even agrees to marry him if it means saving the human race).