I like the idea of having to catch lightning. Thats pretty cool. I would really like to see more of the ecology of this steel. Does it exist naturally, if it does: what is it like there? Are there any myths or rumors about it, or famous people who used it to do something great (even if untrue)? What is some cool local folklore about it? Is it hard to mine or purify, and what special precautions have to be taken by what group of people. Does it affect local trade anywhere. Can it be used in any alchemical process. Frankensteins monster?
Little bits of detail like that really take a solid idea, and make it a must have idea.
The evil shark people link is broken, but I like this idea, but I just happen to like the idea of whales as being more powerful and a threat to humanity, rather than it's victim. I can see the black leviathans coexisting in varying states of peace and war with things like Cheka's krakens.