I enjoyed learning about the process-- how Jackson would say, you're all doing the same thing, go wild-- and they would make all these crazy designs like having the Watcher in the Water being a giant seal-- and then they would go back to almost what they started with, but pulling in a few of their favorite details from the wild designs. I think that's a good way to do creative work.
I liked how respectful they were of the books in their designs.
Some of the most interesting designs were for the Easterlings, who you barely notice in the movie.
I liked how the armorers would come to the artists and say, look, you can draw this stuff, but you can't actually wear it in combat, the joints won't bend the right way, these spikes will poke the wearer... it made it so that there was a reality to the armor fantasy drawings often are missing.
I always like the unused examples in these books-- it makes me imagine an entire alternate movie, where everything is a little different.