Because of the conflict with Gimmick, which is used for a concept not quite congruent to this one. Suggested alternatives: Characterization Schtick (or maybe just "Schtick"), Defining Trait.
Topic on Talk:Signature Trait
Defining Trait or Signature Trait could work.
I'm with "Signature Trait", with "Schtick" as synonym.
I'm liking the sound of "Signature Trait".
Okay, then, if no one has had a problem with it in two months, then I'm going to go ahead and make the move.