Since this is the first issue, the Watcher goes into a little more detail about parallel universes and alternate futures, going so far as to mention whatever crossover universe allowed Superman versus Spider-Man!
But back to the issue at hand. Surely asking what would have happened if Spider-Man had joined the Fantastic Four couldn't be answered with a high body count... could it? The subject matter seems inocuous enough...
What If? Vol.1 #1 (Feb 1976)
Based on: Amazing Spider-Man #1, and events from ASM #2, and Fantastic Four #13-14.
The true history: Spidey tried to join the FF because he thought they paid well. Since they gave all their cash away to scientific research (i.e. Reed blew it all on his hobbies), he didn't take the job. That, and they were kind of mean to him, buying into J.J.J.'s propaganda.
Turning point: What if Sue Richards spoke up for Spider-Man?
Story type: Hockey trade.
Watcher's mood: Musclebound zombie.
Altered history: Peter Parker unmasks to his new team, the Fantastic Five. Endorsed by Reed Richards, J.J.J. seems to jump on the Spidey-loving band wagon. The Chameleon who should have fought Spider-Man in his first issue instead "lapses into relative obscurity" (oh wait, that happened here too). The new team goes on to face the threats they all would have, but together, as Spider-Man commits his first fashion faux pas by putting a little 5 in his chest spider, and Sue gets to sit out all the cool missions.
But when the Puppet Master makes Namor kidnap her, she winds up choosing him over Reed. Hey, the F.F. doesn't need her, right? Not with the more powerful Spider-Man. So she goes through the DNA-changing machine and becomes Namor's water-breathing Atlantean queen, which Reed hopes will keep the Sub-Mariner mellow.
Books canceled as a result: Well, it doesn't look like an Amazing Spider-Man title would be necessary, but if the Human Torch could get his own solo book in the 60s, so could Spidey. No doubt he was the first to go exclusively solo and be replaced by Medusa or Luke Cage. At least Fantastic Four doesn't have to be retitled. They go back to 4 in a matter of a few issues. No Franklin Richards specials down the line though.
These things happen: Was Spider-Man EVER a member of the FF in the standard Marvel continuity? No, though he was a frequent ally of the Torch's. The Fantastic Four DID become the Fantastic Five, but in the so-called MC2 universe, Spider-Girl's future timeline, which spun out of What If? Vol.2 #105. A middle-aged Spider-Man was not part of its line-up. [De rightly corrects me that "he did team up with Wolverine, Ghost Rider, and the Hulk as a Fantastic Four in FF #347-349. So there.]
So not a massacre. That'll have to wait for a later issue. Refreshing in hindsight, though the changes made don't result in a dramatically different timeline, which is somehow less than satisfying.