Probably the biggest recurring plot point he liked to use was ancient aliens, or god-like beings tied to advanced/alien technology. There were The Kree planting their Sentry robots on Earth and turning early homo sapiens into The Inhumans, The Celestials also tampering with humans to create The Eternals, the whole idea of The New Gods essentially using technology and superhero tropes to fill a pantheon, the scientists who attempted to breed the perfect being, only for HIM (later renamed Adam Warlock by Roy Thomas) to be a messiah figure beyond their comprehension and control, many of his later depictions of Asgard became more technologically based, and even a majority of aliens like Galactus, The Silver Surfer and The Watcher essentially being like humans, only from a great beyond, and more powerful than any human could possibly imagine.
Of course, he also did a lot of more grounded work, dealing with major concerns from the early 40s to mids 70s. Very anti-Hitler, anti-atomic testing, anti-red scare, pro-civil rights.
The characters themselves tended to be either incredibly fit (Captain America, Thor, Nick Fury, Black Panther, OMAC, HIM, most of New Genesis), freakishly bizarre (Fing Fang Foom, The Thing, The Watcher, Groot, Ego the Living Planet), or some combination of the two (The Hulk, The Beast, Galactus, most of The Inhumans and Apokolpis). With the more fit characters often appearing as straightforward positive role models, while the more freakish tended to still be sympathetic, but feared, misunderstood, and hated by society. Though a lot of thatcould also come from Stan Lee.
And a few unique naming conventions, the easiest to spot being that groups often had a rare adjective used as a noun, with a "The" in front.
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