Ok, to answer the original poster's question, Rachel probably does live a little longer than the other replicants simply because Tyrell made her to be 'special'.

The film deals with a lot of big themes including:

-What it really means to be 'human'

-If we meet our maker, what would we ask him/her/it?

-Slavery

There's also a huge debate about wether Deckard is a replicant. The Director, Ridley Scott says 'Yes' but Harrison Ford says 'No'.

We could also go into the film-school stuff about how each major character has an animal motif -Zora with her snakes, Roy with his doves, Tyrell and his owl etc...but let's not, eh?