The Elseworlds series allows DC Comics writers and artists the chance to explore storylines and ideas that wouldn't normally be allowed under normal continuity. Bat Man can become a vampire, Superman can be raised by Darkseid, that sort of thing.
JLA - The Nail puts forward the idea of what could happen if Kal-El, the last son of Krypton, was never found by Jonathon and Martha Kent all those years ago. He never grew up in Smallville, never moved to Metropolis, and susequently, Lex Luthor's rise to power went unabated.
And thus we have our story. The JLA never had Superman's steadying influence, and with Lex Luthor leading the way, and borrowing an idea that has been the life-blood of Stan Lee's "X-Men" series for years, super-heroes, or meta-humans, are becoming distrusted by mankind.
The story is well thought out, and carefully constructed. Superman does make an appearance, but I won't spoil it for you by saying in what capacity.
If I have one criticism of this story is that it's a little too short. I would have loved to see a little more in depth detail with regards to certain events. But that being said, it is well written and well constructed. If like me you like to see normal fictional timelines screwed around with a little, then this is right up your alley, as other Elseworld offerings will be.