There's been some discussion and varying opinions on what is acceptable reflavoring and what isn't. Most of the examples have been about reflavoring animal companions (on which the consensus seems to have been negative) or equipment (on which the consensus seems to have been more accepting). As always, reflavoring cannot give mechanical benefits, and the GM always has the veto right to say that the reflavor is nullified for their table. And, of course, Expect Table Variation (tm).
With that in mind, how would you feel about someone reflavoring their Alternate Racial Trait? Most of them have some in-game flavor attached to them. For example, the Darkvision for gnomes:
Some gnome strains have lived in the underground depths for so long they have given up on light entirely (...)
Can this flavor be hand-waived for my metropolitan gnome, whose backstory mentions nothing about living underground? Could it reflavored by explaining that he often stays awake into the wee hours?
Or the Heart of the Slums for humans:
Humans who eke out a life in a city's teeming slums must be quick and clever.
Can this flavor be hand-waived for my upper-class human, whose backstory mentions nothing about living in the slums? Could it be reflavored by explaining that he's otherwise clever and dealt with diseases?