Tom Breevort has an interesting collection of Kirby's covers over at Marvel.com and makes a number of small but interesting comments on each cover as you scroll down.
Interestingly enough we get to see the design sense of Kirby down through the ages, ones that likes a certain number of motifs and layouts, and reuses them many times. Obviously Kirby relied on a short hand in his work, similar figures, similar layouts, and it is this shorthand that most people draw when they do a kirby homage or parody, but it was a shorthand that he created over all those years of work.
Kirby's covers were very often masterpieces of having more than one level to them, so that even the most static told a story, or the teaser of a story, that made you want to decide to pick the book up. Many of the same static poses used today on covers are simply that, static, without life or energy. I sometimes wonder that artists haven't figured out that you can use story elements as compositional devices that will give you an exciting cover, a good pin-up piece, and something to grab the reader's eye. Is this truly brain surgery?
It is interesting to note that many of the Kirby covers lean left, and I wonder if this is because, as many folks have mentioned, that Jack would start at the left corner of the cover and and work his way right? Maybe he just thought out the cool elements and started over there and went further right. Perhaps it was because he knew, knew, that Stan was going to put a hyperbolic caption somewhere on the cover, and he had to leave some negative area for the lettering.
Perhaps we should ahve a Jack Kirby Collector style article with all the rejected covers that he did, alongside the ones that ran, so we could try to get a handle on the rejected material as well. That might prove interesting.