I just re-read What If #1. What a treasure trove for new readers.
Introduces the Watcher
Teases the Avengers, Invaders, Squadron Supreme, Deathlok, and Killraven
Recaps origin of the Fantastic Four
Recaps origin of Spider-Man
Recaps Amazing Spider-Man #1
Introduces the multi-verse
Introduces J. Jonah Jameson, Chameleon, and Vulture
Describes the events in FF #13 (Red Ghost, Blue Area of the Moon, Watcher)
Describes the events in FF #14 (Sub-Mariner, Puppet Master)
Describes the worst case of Stockholm Syndrome in comics