The world of Rapture as it appeared in the first BioShock
may have seemed totally complete, but Irrational Games revealed some elements that were kept behind the development curtain from us -- basically, monsters. The company revealed some rejected enemy designs from its "Vault," including some mutants that were much more ... mutated than the Splicers we saw in the final game.
We imagine that people wouldn't look back so fondly on their experience in Rapture if it had been populated by melting guys and people that were in the process of turning into giant bugs. Creepy little girls seem just a bit less creepy in comparison.