The Kirby style most people are familiar with is the superflat 70s and 80s Kirby. At least that's the style people generally imitate when they draw in his style.
I love his 60s Marvel superheroes, not just because of his iconic surface stylings, but because of how solid his drawings were. You can see that even better in his earlier comics.
Kirby is known for making up muscles that don't exist, but the forms the surface muscles lay on tell me he actually has a very good feel for real anatomy.
Look at the beautiful and solid shapes of Thor's arms. Absolutely amazing! -and the upshot of the barrel chest. Very convincing.
He drew great hands too - and they weren't flat and squared off like his imitators draw them.
I think Kirby is not merely a stylist, but a fantastic draftsman who must have studied and drew everything he saw all the time. The guy could draw scenes in any locale, in every genre and from any angle. -And he had to do tons of these drawings every day because he was a comic book artist. By comparison, a magazine illustrator had a leisurely job. He could pose and copy live models and spend a lot of time on a single scene.
Ger Apeldoorn has been posting a lot of 50s Kirby art from DC comics and you can really see how solid and studied his skills were before he settled into what is known today as the Kirby style.