Not "what" but "who", the who being the typical ten year old enticed to learn why Krypto was delivering the mail.
We're talking about covers, not storylines though. I've waxed eloquent about Marvel's competitive strengths, i.e. Stan Lee, in other places such as here:
The covers though were not among Marvel's strengths in the early to mid 1960's. It was the covers that attracted me to DC comics in 1962 and kept me buying DCs until 1965. DC covers were slick. Marvels (and Charltons) looked low-end-of-the-market in comparison.