1966 saw a lot of changes to the artistic line-up. This is the year Kirby really broke through, in my opinion; previous to this I'd been wondering what all the fuss was about.
Spider-Man leaves Kirby's hands for those of Rick Romita, Sr. We get a slew of relatively intricate, family-dependent plots. Word balloons start overtaking the panels, but overall it's a good year that sees a lot of world-building, and the introduction of Mary Jane Watson, for good or ill.
Buscema joins the Tales to Astonish crew as that book disintegrates in confusion (literally, the archives are full of holes).
Journey Into Mystery has Thor sharing his true identity with Jane, gets a title change to Thor which will play HAVOC with my screen shot archiving system, and descends into an annoying slog with Hercules. It gets back on track with the expedition to Rigel and Ego, the Living Planet, and Kirby is in brilliant form.
Gene Colan takes over drawing Iron Man's half of Tales of Suspense. I like Colan's work. There is a pleasing amount of angst and stabs toward characterization. Captain America is still trying to find a direction; they tease a love interest in the past and possibly one in the future, and connect him up with Nick Fury only to scotch the idea of Cap working for SHIELD, but at least he's got something to do other than punch Nazis.
The Avengers get political. The team dynamic seems to be settling down, though the roster remains unstable. Hank Pym reappears as Goliath, and is still difficult to like. Pietro and Wanda spend much of the year absent. Black Widow is kind of an ally now.
There are lots of introductions in the Fantastic Four, with the establishment of the Inhumans and the iconic milestone that is Galactus (argh). Kirby does a lot of good stuff, and in the second half of the year they try some more intricate multi-threaded plotting, which works well with such a large cast. They introduce the Black Panther and Johnny's roommate Wyatt in a decent-for-the-times stab at diversity, but the writing of Sue is just awful, and Alicia's only appearance is to play midwife to the Surfer's appreciation of humanity.
The X-Men are handed over to Jay Gavin and continue to meander along. Jean goes to college. It's not horrible, but there's nothing interesting about it.
Overall, plots are stretching out a bit more. The universe as a whole gets bigger with the addition of Olympus and the fleshing-out of the Inhumans adding yet another branch on the super-powers tree (to go along with radiation/chemically-enhanced humans, magic, and mutants), and the exploration of sub-space. There is an uptick in the acceptable level of violence, where you get more of the bad guys mowing down innocent villagers, or the Red Skull mind-controlling a guy into shooting himself.