This is the year where the Marvel Universe truly becomes a Universe. It's 'announced' more or less, with the big FF/Avengers/Hulk fight in FF #25-26, and then goes on with a ton of guest appearances and very tight continuity across books. This is what this whole site is about, and this is where it really starts to happen.

If Paste Pot Pete gets an early parole because he helped the Avengers dissolve Baron Zemo's Adhesive X, then when he next shows up to bother the Human Torch, he talks about it. It's now a part of his history. This sounds like a small thing, but it's one of the main things that made Marvel something special in the comic book industry in the mid-60s (the other thing was their 'real life' characterizations of their characters). The very fact that the Avengers would think to call up a super villian from another comic book to ask for help is a huge deal.

'Regular' crossovers, and villain cross-pollination really picks up: Magneto vs. Thor, Loki vs. Dr. Strange, X-Men & the FF vs. the Puppet Master & the Mad Thinker, Spider-Man & Daredevil vs. the Ringmaster, Kraven and the Chameleon vs. Iron Man

Some super-villain teams and partnerships form this year: the Brotherhood of Evil Mutants, the Masters of Evil, Cobra and Hyde, the Sinister Six.

This is still mainly a Lee/Kirby/Dikto show, with Don Heck on Iron Man and Dick Ayers and others occasionally popping up in a fill-in capacity. Lots of continuity inserts: Busiek's Untold Tales of Spider-Man is still going strong, and the second part of Iron Man: The Iron Age takes place here, although it's not as great as the first one. Mark Waid's Captain America: Sentinel of Liberty provides a very nice look at Cap's adjustment to the modern world. Frank Miller's Man Without Fear mini provides a great origin for Daredevil, and a -1 Flashback book for Electra by Peter Milligan fits in here as well.

Some of this year's first appearances: Daredevil, Blob, Scarecrow, Mastermind, Toad, Scarlet Witch, Quicksilver, Electro, Kraven the Hunter, Grey Gargoyle, Beetle, Enchantress, Executioner, Baron Zemo I, Black Widow, Mysterio, Green Goblin, Owl, Hawkeye, Kang (as Kang), Immortus, Unus the Untouchable

Amazing Spider-Man #8  
Avengers #2-4 1st Captain America in modern age. 1st Space Phantom. 
Untold Tales of Spider-Man #4  
Uncanny X-Men #3 1st Blob 
Fantastic Four #22 Invisible Girl's powers expand: projected invisibility, forcefields 
Amazing Spider-Man #9 1st Electro 
Fantastic Four #23  
Tales of Suspense #51 1st Scarecrow 
Uncanny X-Men #4 1st Brotherhood of Evil Mutants (Magneto, Scarlet Witch, Quicksilver, Toad, Mastermind) 
Captain America: Sentinel of Liberty #5-6  
Daredevil: The Man Without Fear #1-5 1st Daredevil, Kingpin, & Elektra. 1st Hand. 
Elektra #-1  
Iron Man: The Iron Age #2  
Tales To Astonish #53  
Amazing Spider-Man #10 ~ Untold Tales of Spider-Man #5-7  
Fantastic Four #24  
Strange Tales #119 (Dr. Strange)  
Untold Tales of Spider-Man #8  
Untold Tales of Spider-Man #9  
Amazing Spider-Man #11  
Untold Tales of Spider-Man #10  
Journey Into Mystery #101-103 1st Enchantress & Executioner 
Tales of Suspense #52 (Iron Man) 1st Black Widow 
Tales of Suspense #52 (Watcher)  
Amazing Spider-Man #12  
Untold Tales of Spider-Man #11  
Untold Tales of Spider-Man #12  
Untold Tales of Spider-Man #13  
Fantastic Four #25-26 Early crossover event: Hulk vs. the Fantastic Four & the Avengers! 
Avengers #5-6 1st Masters of Evil (Baron Zemo I, Black Knight, Melter, Radioactive Man). 
Tales of Suspense #53  
Uncanny X-Men #5  
Strange Tales #120  
Journey Into Mystery #104-106 1st Cobra & Hyde team-up 
Strange Tales #121  
Amazing Spider-Man #13 1st Mysterio 
Fantastic Four #27  
Untold Tales of Spider-Man #14  
Tales of Suspense #54 (Watcher)  
Daredevil #2  
Tales of Suspense #55  
Amazing Spider-Man #14 1st Green Goblin 
Strange Tales #122  
Fantastic Four #28  
Untold Tales of Spider-Man #15  
Untold Tales of Spider-Man #16  
Uncanny X-Men #6  
Amazing Spider-Man #15 1st Kraven the Hunter 
Tales to Astonish #58  
Strange Tales #123 1st Beetle 
Daredevil #3 1st Owl 
Tales of Suspense #56 (Watcher)  
Fantastic Four #29  
Journey into Mystery #107-108 1st Grey Gargoyle 
Avengers #7  
Tales of Suspense #57 1st Hawkeye 
Tales to Astonish #59  
Strange Tales #124  
Untold Tales of Spider-Man #17  
Journey Into Mystery #109  
Uncanny X-Men #7  
Fantastic Four #30 1st Diablo 
Untold Tales of Spider-Man: Strange Encounter  
Avengers #8 1st Kang 
Amazing Spider-Man #16  
Strange Tales #125 (Dr. Strange)  
Fantastic Four #31  
Untold Tales Of Spider-Man #18  
Amazing Spider-Man annual #1 1st Sinister Six (Doctor Octopus, Electro, Kraven, Mysterio, Sandman, Vulture) 
Untold Tales Of Spider-Man #19  
Amazing Spider-Man #17  
Tales to Astonish #60 (Hulk)  
Tales of Suspense #58 (Iron Man)  
Tales of Suspense #58 (Watcher)  
Daredevil #4 1st Purple Man 
Avengers #9 1st Wonder Man 
Strange Tales #126-127 (Dr. Strange)  
Amazing Spider-Man #18 ~ Untold Tales of Spider-Man #20-22  
Uncanny X-Men #8 1st Unus 
Tales Of Suspense #59 (Iron Man)  
Tales of Suspense #59 (Captain America)  
Journey Into Mystery #110-111  
Avengers #10 1st Immortus 
Fantastic Four #32  
Amazing Spider-Man #19  
Fantastic Four Annual #2 (Dr. Doom origin excerpt)  
Tales of Suspense #60  
Untold Tales of Spider-Man '97  
Uncanny X-Men #9  
Tales of Suspense #62 (Iron Man)  
Avengers #11  
Daredevil #5  
Fantastic Four #33 1st Attuma